an aquarium tank can hold 7700 liters of water. There are two pipes that can be used to fill the tank. The first pipe alone can fill the tank in 44 minutes. The second pipe can fill the tank in 77 minutes by itself. When both pipes are working together, how long does it take them to fill the tank?

Pipe 1 works at a rate of (1 tank) / (44 min) = 1/44 tank/min.

Pipe 2 works at a rate of (1 tank) / (77 min) = 1/77 tank/min.

Together, both pipes work at a rate of (1/44 + 1/77) tank/min = 1/28 tank/min.

In this context, 1 tank = 7700 L. So when both pipes are working, water is flowing into the tank at a rate of

(1/28 tank/min) * (7700 L/tank) = 7700/28 L/min = 275 L/min

It then stands to reason that it would take 28 min for both pipes to fill the tank, since

(275 L/min) * (28 min) = 7700 L