two pipes a and b can fill a cistern in 12 and 15 minutes respectively . both are opened together but after 4 minutes a is turned off . after how much more time will the cistern be filled ?
"4 / 12 + ( 4 + x ) / 15 = 1 x = 6 answer : a"
add(multiply(12, subtract(const_1, multiply(add(inverse(12), inverse(15)), 4))), 4)