by selling 12 pens for a rupee a woman loses 20 % . how many for a rupee should he sell in order to gain 20 % ?
"d 80 % - - - 12 120 % - - - ? 80 / 120 * 12 = 8"
inverse(add(divide(divide(const_100, subtract(const_100, 20)), 12), divide(multiply(divide(divide(const_100, subtract(const_100, 20)), 12), 20), const_100)))