pipe p can fill a tank in 3 hours , pipe q in 9 hours and pipe r in 18 hours . if all the pipes are open , in how many hours will the tank be filled ?
"explanation : part filled by ( p + q + r ) in 1 hour = ( 1 / 3 + 1 / 9 + 1 / 18 ) = 1 / 2 all the three pipes together will fill the tank = 2 / 1 = 2 hours answer a"
inverse(add(divide(const_1, 18), add(divide(const_1, 3), divide(const_1, 9))))