two pipes p and q can fill a cistern in 12 and 13 minutes respectively . both are opened together , but at the end of 3 minutes the first is turned off . how much longer will the cistern take to fill ?
"3 / 12 + x / 13 = 1 x = 7 3 / 4 answer : e"
multiply(subtract(const_1, multiply(add(divide(const_1, 12), divide(const_1, 13)), 3)), 13)