by selling 80 pens , a trader gains the cost of 20 pens . find his gain percentage ?
"let the cp of each pen be rs . 1 . cp of 80 pens = rs . 80 profit = cost of 20 pens = rs . 20 profit % = 20 / 80 * 100 = 25 % answer : a"
multiply(divide(20, 80), const_100)