two pipes a and b can fill a cistern in 9 and 18 minutes respectively , and a third pipe c can empty it in 24 minutes . how long will it take to fill the cistern if all the three are opened at the same time ?
"1 / 9 + 1 / 18 - 1 / 24 = 1 / 8 8 / 1 = 8 answer : c"
add(multiply(9, subtract(const_1, multiply(add(inverse(9), inverse(18)), 24))), 24)