A swimming pool can be filled in 13 hours if water enters through a pipe alone, or in 21 hours if water enters through a hose alone.

Question

If water is entering through both the pipe and the hose, how long will it take to fill the pool?

Answer 1

In 1hr pipe will fill 1/13th of pool

In 1hr hose will fill 1/21th of pool

 

If both are filling the pool

 

In 1 hr 1/13 + 1/21 = 34/273th of the pool will be filled

 

or

 

34/273th pool will be filled in 1 hr

1 pool will be filled in 1/(34/273) hr = 273/34 hr = 8 hrs

:)