by selling 100 pens , a trader gains the cost of 40 pens . find his gain percentage ?
"let the cp of each pen be rs . 1 . cp of 100 pens = rs . 100 profit = cost of 40 pens = rs . 40 profit % = 40 / 100 * 100 = 40 % answer : a"
multiply(divide(40, 100), const_100)