by selling 90 pens , a trader gains the cost of 30 pens . find his gain percentage ?
"let the cp of each pen be rs . 1 . cp of 90 pens = rs . 90 profit = cost of 30 pens = rs . 30 profit % = 30 / 90 * 100 = 33.3 % answer : a"
multiply(divide(30, 90), const_100)