$ 500 will become $ 1000 in 5 years find the rate of interest ?
si = simple interest = a - p = 1000 - 500 = $ 500 r = 100 si / pt = 100 * 500 / 500 * 5 = 20 % answer is b
divide(subtract(1000, 500), multiply(500, divide(5, const_100)))