by selling 15 pencils for a rupee a man loses 20 % . how many for a rupee should he sell in order to gain 20 % ?
"80 % - - - 15 120 % - - - ? 80 / 120 * 15 = 10 answer : e"
multiply(divide(const_1, multiply(add(const_100, 20), divide(const_1, subtract(const_100, 20)))), 15)