To add 5 and 7, you can count on your fingers or use a mental math strategy. Starting with 5, count up 7: 5, 6, 7, 8, 9, 10, 11, 12. Therefore, the answer is 12.

To subtract 13 from 20, you can use the counting down method. Starting at 20, count down 13: 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7. Therefore, the answer is 7.

To multiply 4 and 9, you can use the traditional multiplication algorithm. Multiply 4 by 9, starting from the rightmost digit: 4 x 9 = 36. Therefore, the answer is 36.

To divide 24 by 6, you can use the division algorithm. Divide 24 by 6, starting from the leftmost digit: 24 ÷ 6 = 4. Therefore, the answer is 4.

To find the value of 10^2, you can multiply 10 by itself: 10 x 10 = 100. Therefore, the answer is 100.

To simplify the expression, follow the order of operations (parentheses first): 8 - (3 + 2) = 8 - 5 = 3. Therefore, the answer is 3.

To solve the equation, isolate the variable x on one side: x = 12 - 5 = 7. Therefore, the answer is 7.

To find the area of a rectangle, multiply the length by the width: Area = length x width = 8 cm x 6 cm = 48 cm^2. Therefore, the answer is 48 cm^2.

To find the perimeter of a square, multiply the side length by 4: Perimeter = 4 x side length = 4 x 10 cm = 40 cm. Therefore, the answer is 40 cm.

To find the volume of a cube, cube the side length: Volume = side length^3 = 5 cm^3 = 125 cm^3. Therefore, the answer is 125 cm^3.

To simplify the expression, combine like terms: (2x + 3y) - (x - 2y) = 2x + 3y - x + 2y = x + 5y. Therefore, the answer is x + 5y.

To solve the equation, isolate the variable x on one side: 3(x - 4) = 15 => x - 4 = 5 => x = 9. Therefore, the answer is 11.

To find the area of a circle, use the formula: Area = πr^2. Substitute the radius r = 7 cm: Area = π(7 cm)^2 = 147π cm^2. Therefore, the answer is 147π cm^2.

To find the volume of a sphere, use the formula: Volume = (4/3)πr^3. Substitute the radius r = 10 cm: Volume = (4/3)π(10 cm)^3 = 4/3π(1000) cm^3. Therefore, the answer is 4/3π(1000) cm^3.

To simplify the expression, combine like terms: (x^2 + 2x + 1) - (x^2 - 3x + 2) = x^2 + 2x + 1 - x^2 + 3x - 2 = 5x - 1. Therefore, the answer is 5x - 1.