$ 600 will become $ 720 in 4 years find the rate of interest ?
"si = simple interest = a - p = 720 - 600 = $ 120 r = 100 si / pt = 100 * 120 / 600 * 4 = 5 % answer is c"
divide(subtract(720, 600), multiply(600, divide(4, const_100)))