two pipes a and b can fill a cistern in 20 and 30 minutes respectively , and a third pipe c can empty it in 60 minutes . how long will it take to fill the cistern if all the three are opened at the same time ?
"1 / 20 + 1 / 30 - 1 / 60 = 4 / 60 60 / 4 = 15 answer : a"
add(multiply(20, subtract(const_1, multiply(add(inverse(20), inverse(30)), const_3))), 60)