find the principal which yields a simple interest of rs . 20 and compound interest of rs . 21 in two years , at the same percent rate per annum ?
"explanation : si in 2 years = rs . 20 , si in 1 year = rs . 10 ci in 2 years = rs . 21 % rate per annum = [ ( ci – si ) / ( si in 1 year ) ] * 100 = [ ( 21 – 20 ) / 20 ] * 100 = 5 % p . a . let the principal be rs . x time = t = 2 years % rate = 5 % p . a . si = ( prt / 100 ) 20 = ( x * 5 * 2 ) / 100 x = rs . 200 answer : d"
divide(multiply(subtract(21, 20), const_100), const_2)