A swimming pools large drain will empty a pool in 3 hours while its smaller drain takes 6 hours. If both drains are open how long will it take to empty the pool?

A: 4 1/2 hours

B: 2 hours

C: 9 hours

D: 4 hours

The answer is C: 9 Hours because you simply add them up together and the total is 9 so, that is the answer.

The large drain empties 1/3 of the pool in 1 hour.

The small drain empties 1/6 of the pool in 1 hour.

When both are open, they drain

(1/3 + 1/6) =

(2/6 + 1/6) =

3/6 = 1/2 of the pool in 1 hour.

1/2 pool/hour is the same unit rate as 2/1 hour/pool.

So both drains working together can empty the pool in 2 hours.