One inlet pipe can fill an empty pool in 4 hours, and a drain can empty the pool in 10 hours. how long will it take the pipe to fill the pool if the drain is left open?

Inlet pipe:

4 hours = 1 pool
1 hour = 1/4 hoor

Drain:

10 hours = 1 pool
1 hour = 1/10 pool

If both are turned on, 
1 hour = 1/4 - 1/10 = 3/20 of the pool.

Number of hours needed to fill the pool if the drain is left open:
1 ÷ 3/20
= 1 x 20/3
= 20/3
= 6 2/3 hours


It will take 6 2/3 hours to fill up the pool.

Ais the correct answer