a small water pump would take 3 hours to fill an empty tank . a larger pump would take 1 / 4 hour to fill the same tank . how many hours would it take both pumps , working at their respective constant rates , to fill the empty tank if they began pumping at the same time ?
rate of the small pump is 1 / 3 tank / hour rate of the larger pump is 1 / ( 1 / 4 ) or 4 tank / hour ; combined rate of the two pumps is 1 / 3 + 4 = 13 / 3 tank / hour , together they will fill the empty tank in 1 / ( 13 / 3 ) = 3 / 13 hours ( time = job / rate ) . answer : a
inverse(add(inverse(3), 4))