a student gets 50 % in one subject , 70 % in the other . to get an overall of 70 % how much should get in third subject .
"let the 3 rd subject % = x 50 + 70 + x = 3 * 70 120 + x = 210 x = 210 - 120 = 90 answer : a"
subtract(multiply(70, const_3), add(50, 70))