i sold a book at a profit of 10 % . had i sold it for $ 90 more , 15 % would have been gained . find the cost price ?
"115 % of cost - 110 % of cost = $ 90 5 % of cost = $ 90 cost = 90 * 100 / 5 = $ 1800 answer is a"
divide(multiply(divide(multiply(90, 10), subtract(15, 10)), const_100), 10)