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Rule of three - class-VIII

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A man eats $200:g$ of rice a day and he has enough rice to last him for $35$ days. How long would the stock of rice last him if he were to eat $250:g$ rice a day.

  1. $\;42$

  2. $\;21$

  3. $\;28$

  4. $\;49$


Correct Option: C
Explanation:

Man eat $ 200$ g rice in a day then he  has $35\times 200=7000  g $ rice for  $35$  days
But if he eat  $250$  g rice in a day
Then $7000$  g rice is enough for $\dfrac{7000}{250}=28  \ days$

The price of $357$ mangoes is $Rs. 1517.25.$ What will be the approximate price of $49$ dozens of such mangoes?

  1. $Rs. 3000$

  2. $Rs. 3500$

  3. $Rs. 4000$

  4. $Rs. 2500$


Correct Option: D
Explanation:

Let the required price be Rs. x. Then, more mangoes, more price
$\therefore$ $357 : (49 \times 12) :: 1517.25 : x$
$\Rightarrow 357x = (49 \times 12 \times 1517.25)$

$\Rightarrow\, x\, =\, \displaystyle \frac{(49\, \times\, 12\, \times\, 1517.25)}{357}\, =\, 2499$
Hence, the approximate price is Rs. 2500

On a scale of map, $0.6$ cm represents $ 6.6$km. If the distance between the points on the map is  $80.5$  cm, the actualdistance between these points is

  1. $9$ km

  2. $72.5$ km

  3. $190.75$ km

  4. $885.5$ km


Correct Option: D
Explanation:

Let the actural distance be x km. Then, more distance on the map, more is the actual distance. 
$\therefore$ 0.6 : 80.5 :: 6.6 : x
$\Rightarrow\, 0.6x\, =\, 80.5\, \times\, 6.6$
$\Rightarrow\, x\, =\, \displaystyle \frac{80.5\, \times\, 6.6}{0.6}\, \Rightarrow\, x\, =\, 885.5$

If x & y are in direct proportion and if $x=20$ at proportionality constant $=4$, find y.

  1. $2$

  2. $3$

  3. $4$

  4. $5$


Correct Option: D
Explanation:

$\dfrac{x}{y}=K\;\Rightarrow\;\dfrac{20}{y}=4\;\Rightarrow\;y=\dfrac{20}{4}=5$

Which is an example of direct proportion?

  1. More working hours, more earning

  2. More speed of car, less time taken

  3. More no. of workers, less time taken

  4. Less the age of person, more active he is


Correct Option: A
Explanation:

2 quantities say it be $x , y$ are said to be in proportion when the change in value of $x$ , leads to the equal change in value of $y$.

If $x$ increases, $y$ increases proportionally, it is called Direct proportion
i.e if a person works more time, he will earn more which is option A.
in other option, are the examples of inverse proportion
so the answer is Option A 

Observe the values and find the quantities which are in direct proportion
$ \begin{equation}x : \;\;[4\;\;6\;\;8\;\;10] \ y : \;\;[2\;\;3\;\;4\;\;\;5] \ z : \;\;[1\;\;2\;\;3\;\;\;4]\end{equation}$

  1. x & y

  2. y & z

  3. x & z

  4. None of these


Correct Option: A
Explanation:

By definition of direct proportion,

$a \  \alpha \  b$ i.e. $a = Kb$ where $K$ is constant of proportionality
$\therefore \dfrac{a}{b} = K$  ........ equation $1$

From given example, consider
$\dfrac{x}{y} =\dfrac{4}{2} = \dfrac{6}{3} = \dfrac{8}{4} = \dfrac{10}{5} = 2$

where as
$\dfrac{x}{z} = \dfrac{4}{1}\neq\dfrac{6}{2}\neq\dfrac{8}{3}\neq\dfrac{10}{4}$

$\therefore$ $x$ and $y$ obey the equation $1$

Hence $x$ and $y$ are in inverse proportion.
Answer is A

If $a:b=c:d$ then how many of the following statements are true?

  1. $c(a+b)=a(c+d)$

  2. $d(a-b)=b(c-d)$

  3. $(a^{2}+b^{2})(ac-bd)=(a^{2}-b^{2})(ac+bd)$

  4. $(a^{2}-b^{2})(ad-bc)=(a^{2}+b^{2})(ac-bd)$


Correct Option: A
Explanation:

$\dfrac{a}{b}=\dfrac{c}{d}$
$\dfrac{b}{a}=\dfrac{d}{c}$

$\left(1+\dfrac{b}{a}\right)=\left(1+\dfrac{d}{c}\right)$
$\dfrac{(a+b)}{a}=\dfrac{(c+d)}{c}$
$c(a+b)=a(c+d)$

Mark the correct alternative of the following.
Two numbers are in the ration $3 : 5$ and their sum is $96$. The larger number is?

  1. $36$

  2. $42$

  3. $60$

  4. $70$


Correct Option: C
Explanation:

Given two numbers are  in the ratio $3:5$.

Let the numbers are $3x$ and $5x$.
Then according to the problem we get,
$3x+5x=96$
or, $8x=96$
or, $x=12$.
So the largest number is $12\times 5=60$.

If $4$ men or $6$ women earn Rs $360$ in one day, then find how much less does a woman earn in one day than men.

  1. Rs $20$

  2. Rs $30$

  3. Rs $40$

  4. Rs $35$


Correct Option: B
Explanation:

$6$ women earn = Rs $360$ $/day$


$\therefore 1$ woman earns = Rs $\dfrac{360}{6}$ $/day$


                             = Rs $60$ $/day$.
$4$ men earn = Rs $360$ $/day$

$\therefore 1$ man earns = Rs $\dfrac{360}{4}$ $/day$

                             = Rs $90$ $/day$.
$\therefore$ women earn Rs 30 less than man

Eight oranges can be bought for Rs $10.40$, then how many more oranges can be bought for Rs $16.90$?

  1. $5$ oranges

  2. $3$ oranges

  3. $7$ oranges

  4. $2$ oranges


Correct Option: A
Explanation:

Let $x$ oranges  be bought for Rs.$ 16.90.$


Given, eight oranges are bought for Rs.$ 10.40.$

Then, $\dfrac{8}{x}=\dfrac{10.40}{16.90}$

$\Longrightarrow x=\dfrac{16.90\times 8}{10.40}=13$

Then, $(13-8)=5$ more oranges can be bought.

$4$ men or $6$ women earn Rs $360$ in one day. Find how much will $6$ men and $4$ women earn in one day?

  1. Rs $780$

  2. Rs $720$

  3. Rs $760$

  4. Rs $740$


Correct Option: A
Explanation:
$6$ women = Rs $360$/day
$4$ women = Rs $360$/day   
One woman  = $x$ Rs/day 
$x = \dfrac { 360 }{ 6 }$  = Rs $60$
 One man = $\dfrac { 360 }{ 4 }$
  = Rs $90$/day.
So,  $6$ men + $4$ women 
$= 6(90) + 4(60)$
$=$ Rs $540$ + Rs $240$
= Rs $780$.

$A$ can do a piece of work in $10$ days and $B$ in $15$ days. How long will they take together to finish it ? 

  1. $7$ days

  2. $3$ days

  3. $9$ days

  4. $6$ days


Correct Option: D
Explanation:

Work done by A in 1 day $=\dfrac{1}{10}$


Work done by B in 1 day$=\dfrac{1}{15}$

Work done by A and B in 1 day$=\dfrac{1}{10}+\dfrac{1}{15}=\dfrac{25}{150}$

Working together they will complete the work in $\dfrac{150}{25}=6$ days

A sum is divided among four persons in the ratio $3\,\colon\,4\,\colon\,5\,\colon\,8$. If the second largest share is  Rs$\,2500$, what is the total sum?

  1. Rs $10000$

  2. Rs $15000$

  3. Rs $1000$

  4. Rs $1500$


Correct Option: A
Explanation:

The share is divided in the ratio $3:4:5:8$

$\therefore$ the second largest share is $5$.
Second largest share$\,=\displaystyle\frac{5}{(3+4+5+8)} \times$ Total share 
 $=\displaystyle\frac{5}{20}\times $ Total sum $=\displaystyle\frac{1}{4}\times $  Total sum

Given, second largest share $=Rs.2500$ 
 $\therefore \displaystyle\frac{1}{4}\times$ Total sum $Rs.2500$
 $\Rightarrow$ Total sum $=Rs.10,000$.

One litre of water weighs $1$ kg. How many cubic millimetres of water will weigh $0.1$ gram?

  1. $100$ cubic mm.

  2. $150$ cubic mm.

  3. $90$ cubic mm.

  4. $80$ cubic mm.


Correct Option: A
Explanation:

$1$ litre $=1000$ cubic cm of water weighs $1000$ g.
$\therefore\,1000$ g is the weight of $(1000\times1000)$ cubic mm ....$(\because\;1$ cm $=10$ mm)
$\therefore\,0.1$ g is the weight of $\displaystyle\frac{1000\times1000}{1000}\times0.1$ cubic mm $=100 $ cu mm.

The cost of $3$ digital cameras and $5$ cell phones is Rs. $35,290$. What is the cost of $9$ digital cameras and $15$ cell phones?

  1. Rs. $1,68,450$

  2. Rs. $1,79,220$

  3. Rs. $1,05,870$

  4. None of these


Correct Option: C
Explanation:

Cost  of ($3$ digital cameras $+5$ cell phones) $=$Rs $35,290$ 

$\because $ Cost of ($9$ digital cameras $+15$ cell phones) $=3\times$ [cost of( $3$ digital cameras $+5$ cell phones)] 
$=3\times$ Rs. $35,290=$ Rs. $1,05,870$.

A rope makes $260$ rounds of a cylinder with base radius $20$ cm, How many times can it go round a cylinder with base radius $26$ cm?

  1. $130$

  2. $300$

  3. $200$

  4. $150$


Correct Option: C
Explanation:

Circumference of circular base of cylinder is $2\pi R$.

Total length of the rope $ = n(2 \pi R)$ , where $n$ is number of revolutions 
Since length will remain constant
$n _{1} (2 \pi R _{1}) $ = $n _{2} (2 \pi R _{2}) $
$n _{1} = 260 $ , $n _{2} = ?$, $R _{1} = 20 cm$ and $R _{2} = 26$ cm
$ 260 \times20 = 26 \times n _{2}$
$n _{2} = 200$

If 18 pumps can raise 2170 tonnes of water in 10 days, working 7 hours a day then in how many days will 16 pumps raise 1736 tonnes of water, working 9 hours a day ?

  1. 6

  2. 7

  3. 8

  4. 9


Correct Option: B
Explanation:

Let the required number of days be x.
 Then,

Less pumps, More days (Indirect Proportion)


Less water, Less days (Direct Proportion)


More hours / day, Less days (Indirect Proportion)
 Pumps 8 : 9


Weight 1085 : 868
 Hours/day 9 : 7 :: 10 : x
$ (8 \times  1085 \times  9 \times  x) = (9 \times  868 \times  7 \times  10)$
$ x = (9 \times  868 \times  7 \times  10)/(8 \times  1085 \times  9)$
$ x=7 $
Answer (B) 7

If $a : b = 5 : 9$ and $b : c = 4 : 7$ find $a : b : c$

  1. $5:9:\dfrac{63}{4}$

  2. $20:63:36$

  3. $4:36:63$

  4. $20:36:63$


Correct Option: A,D
Explanation:

Given, $a : b = 5 : 9 $ and $b : c = 4 : 7$ $=$ $\displaystyle \left ( 4\times

\frac{9}{4} \right ):\left ( 7\times \frac{9}{4} \right

)=9:\frac{63}{4}$
$\displaystyle \Rightarrow a:b:c=5:9:\frac{63}{4}=20:36:63$

Cost of 10 mangoes is Rs. 100 The cost of 18 mangoes is__ 

  1. Rs. 18

  2. Rs. 108

  3. Rs. 200

  4. Rs. 180


Correct Option: D
Explanation:

The cost of mangoes is directly proportional to the number of mangoes bought.

So, if $ 10 $ mangoes costs $ Rs\  100 $, then $ 1 $ mango will cost $ Rs \dfrac{ 100 }{10}  = Rs  10 $

And $ 18 $ mangoes will cost $ 18 \times Rs \dfrac{ 100 }{10}  = Rs 180 $

14 apples cost Rs. 140 The cost of 1 apple is__

  1. Rs. 10

  2. Rs. 8

  3. Rs. 4

  4. Rs. 14


Correct Option: A
Explanation:

The cost of apples is directly proportional to the number of apples bought.

So, if $ 14 $ apples cost $ Rs\  140 $, then $ 1 $ apple will cost $ Rs \dfrac{ 140 }{14}  =  Rs  10 $

A bus travels 120 km in 4 hrs  The distance that can be travelled by the bus in 6 hrs is__

  1. 18 km

  2. 140 km

  3. 180 km

  4. 120 km


Correct Option: C
Explanation:

The distance travelled is directly proportional to the number of hours travelled.

So, if  in $ 4 $ hours, $ 120\ km $ is travelled then in $ 1 $ hour  distance travelled is $  \dfrac{ 120 }{4}\ km $  

And in  $ 6 $ hours, distance travelled is  $ 6 \times \dfrac{ 120 }{4}  = 180\ km $

If y is directly proportional to x & when $x=2$ & $y=4$, what is constant of proportionality?

  1. $1$

  2. $3$

  3. $5$

  4. $2$


Correct Option: D
Explanation:

By definition of proportionality,

If $y$ is directly proportional to $x$,
$y$ $ \alpha$ $ x$
$\therefore y = Kx$     .......Equation $1$
where $K$ is constant of proprtionality
Now,
Given that $x = 2 , y = 4$
putting the above values in Equation $1$ we get, 
$4 = K\times 2$
$\therefore K = 2$

A drum of kerosene oil is $\dfrac {3}{4}$ full. When $30$ litres of oil are drawn from it, it is $\dfrac {7}{12}$ full. Find the capacity of the drum ?

  1. $120$ litres

  2. $140$ litres

  3. $180$ litres

  4. $240$ litres


Correct Option: C
Explanation:

Let capacity of drum  $=  x$ litres
Initially, drum was $\dfrac {3x}{4}$ part filled
$30$ litres oil drawn then it become, $\dfrac {7x}{12}$ part.
Now, according to question,
$\dfrac {3x}{4} - \dfrac {7x}{12} = 30$
$\Rightarrow \dfrac {(9x - 7x)}{12} = 30$
$\Rightarrow 2x = 360$
$\Rightarrow x = 180$ litres

Which of the following $a$ & $b$ are in direct proportion?

  1. $a=\dfrac{2}{b}$

  2. $a=2b$

  3. $a=b^3$

  4. $a=\dfrac{1}{b^2}$


Correct Option: B
Explanation:
For proportion $a:b=c:d$
$a=2b$
$\Rightarrow a:b=2:1$

The cost of $2$ meter cloth is $10$. Find the cost of $100\ m$ cloth.

  1. $400$

  2. $500$

  3. $1000$

  4. $1500$


Correct Option: B
Explanation:

Cost of cloth is directly proportional to its length.
$10 = 2\times k \Rightarrow k = 5$    , where k is the cost of cloth per meter
$\therefore$ Cost of $100\ m$ cloth $=5\times 100 = 500$.

If x & y are in direct proportion then find the value of a
$\begin{equation}x\;\;2\;\;3\;\;5\;\;\;7 \ y\;\;4\;\;6\;\;a\;\;14\end{equation}$

  1. $8$

  2. $10$

  3. $15$

  4. $20$


Correct Option: B
Explanation:

$\dfrac{x}{y}=K$ (Direct proportion)


$\dfrac{2}{4}=\dfrac{3}{6}=\dfrac{5}{a}=\dfrac{7}{14}=K\;\Rightarrow\;K=\dfrac{1}{2}=\dfrac{5}{a}\;\Rightarrow\;a=10$

Thus option B is the correct answer.

Find the value of $x$ if  $a$ and $ b$  are in direct proportion

  $a$ $2$ $3$ $4$ $5$
  $b$  $14$ $21$ $x$ $35$
  1. $16$

  2. $25$

  3. $27$

  4. $28$


Correct Option: D
Explanation:

$a$ and $b$ are in direct proportion
$\therefore \dfrac {a}{b} = k = \dfrac {2}{14} = \dfrac {3}{21} = \dfrac {1}{7}$
$\therefore \dfrac {4}{x} = \dfrac {1}{7} \Rightarrow x = 28$.

$a$ and $b$ are in ............ proportion

$a$ $3$ $7$ $10$ $11$
$b$ $9$ $21$ $30$ $33$
  1. direct

  2. indirect

  3. both

  4. none


Correct Option: A
Explanation:

$\dfrac {a}{b} = \dfrac {3}{9} = \dfrac {7}{21} = \dfrac {10}{30} = \dfrac {11}{33} = \dfrac {1}{3} = k$
$\therefore a$ and $b$ are in direct proportion.

If  $a$  is inversely proportional to  $b $ and  $b$  is inversely proportional to  $c $ then what is proportionality between  $a $ and  $c$?

  1. Direct

  2. Inverse

  3. No proportionality

  4. Can't be determinal


Correct Option: A
Explanation:

$a\propto \dfrac {1}{b} \Rightarrow a = \dfrac {k _{1}}{b}; k _{1}$ is a constant
$b\propto \dfrac {1}{c} \Rightarrow b = \dfrac {K _{2}}{c}; k _{2}$ is a constant
$\Rightarrow a = \dfrac {k _{1}c}{k _{2}} = k _{3}c; k _{3} = \dfrac {k _{1}}{k _{2}}$ is another constant
$\Rightarrow a\propto c$

The correct dosage of adult over-the-counter medicine a child can receive is given by a formula by Clark. The child's weight, in pounds, is divided by $150$, and the result is multi pounds lied by the adult dose of the medicine. A mother need to give her daughter acetaminophen, which has an adult dose of $ 1000$ milligrams. She does not know her daughter's exact weight, but she knows the weight is  and between $75 $ and $90 $pounds. Find the range of correct dosage, d, in milligrams of acetaminophen the daughter could receive.

  1. $50$

  2. $500$

  3. $1000$

  4. $1600$


Correct Option: B

$A, B$ and $C$ can finish a job working alone in $72, 24$ and $36$ days respectively. In how many days they can finish the job if they worked together?

  1. $12$

  2. $9$

  3. $15$

  4. $18$


Correct Option: A
Explanation:

Let the total work be $72$ units (LCM on $72, 24$ and $36$).


$A, B$ and $C's$ one day work is $1, 3$ and $2$ units respectively.

Required number of days $= \dfrac {72}{6} = 12$.


Alternate method
$(A+B+C)'s$ one day work =$\dfrac{1}{72}+\dfrac{1}{24}+\dfrac{1}{36}$

$=\dfrac{1+2+3}{72}=\dfrac{6}{72}$

Number of days required $= \dfrac {72}{6} = 12$ days to finish the work when 3 of them work together.

If $4$ men earn Rs $360$ in one day, then how much does a man earn in one day?

  1. $90$

  2. $30$

  3. $120$

  4. $60$


Correct Option: A
Explanation:

Earning of $4$ men $=$ Rs $360$ per day
Earning of $1$ man $=$ Rs $\dfrac{360}{4}$ per day

                             $=$ Rs $90$ per day

Which of the following is the example of direct proportion?

  1. Number of mangoes in a bag and weight of the bag.

  2. Speed goes up ,travel times goes down.

  3. More the number of men lesser the time taken to complete it.

  4. None of these.


Correct Option: A
Explanation:

Directly proportional: as one amount increases, 
another amount increases at the same rate.
Hence, in option A when number of mangoes in a bag increases,then the weight of the bag also increases.

Share of A, B and C respectively, are ____________, if Rs. $5460$ is divided in $\displaystyle\frac{1}{2}:\frac{1}{3}:\frac{1}{4}$.

  1. Rs. $1680$, Rs. $2520$, Rs. $1260$

  2. Rs. $2520$, Rs. $1680$, Rs. $1260$

  3. Rs. $1260$, Rs. $2100$, Rs. $2520$

  4. Rs. $2520$, Rs. $1260$, Rs. $1680$


Correct Option: B
Explanation:
Let A's share $=Rs.\left(\displaystyle\frac{x}{2}\right)$
B's share $=Rs.\left(\displaystyle\frac{x}{3}\right)$
And C's share $=Rs.\left(\displaystyle\frac{x}{4}\right)$
According to equation,
$\displaystyle\frac{x}{2}+\frac{x}{3}+\frac{x}{4}=5460$
$\Rightarrow \displaystyle\frac{6x+4x+3x}{12}=5460$
$\Rightarrow 13x=5460\times 12\Rightarrow x=\displaystyle \frac{5460\times 12}{13}=5040$
$\therefore$ A's share $=Rs. \left(\displaystyle\frac{5040}{2}\right)=Rs. 2520$
B's share$=Rs.\left(\displaystyle\frac{5040}{3}\right)=Rs. 1680$
And C's share$=Rs. \left(\displaystyle\frac{5040}{4}\right)=Rs. 1260$.

If $20: 28= x:7=10:y$.
The values of $x$ and $y$ in the box respectively are __________.

  1. $5, 14$

  2. $14, 5$

  3. $8, 10$

  4. $10, 8$


Correct Option: A
Explanation:
We have, $20:28=x:7=10:y$
Taking first two ratios, we have
$20:28=x:7$
$\Rightarrow 20\times 7=x\times 28$
$\Rightarrow x=\displaystyle\frac{20\times 7}{28}=5$
Again taking last and first ratio, we get
$20:28=10:y\Rightarrow 20\times y=28\times 10$
$\Rightarrow y=\displaystyle\frac{10\times 28}{20}=14$.
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