[Q.11] A sum of money doubles itself in 8 years under simple interest. What is the rate of interest per annum?
SI = P => P * R * 8 / 100 = P => R = 100 / 8 = 12.5%.
[Q.12] What is the area of a circle with a radius of 7 cm? (Take pi = 22/7)
Area = pi * $r^2$ = (22/7) * 7 * 7 = 154 $cm^2$.
[Q.13] If x + 1/x = 4, what is the value of $x^2 + 1/x^2$?
(x + 1/$x)^2$ = $x^2 + 1/x^2 + 2$ = 16 => $x^2 + 1/x^2$ = 14.
[Q.14] Two numbers are in the ratio 5 : 7. If their difference is 12, find the larger number.
7x - 5x = 2x = 12 => x = 6. Larger number = 7 * 6 = 42.
[Q.15] A shopkeeper offers a discount of 20% on the marked price of Rs. 1,500. What is the selling price?
Discount = 20% of 1500 = 300. SP = 1500 - 300 = Rs. 1,200.