Question

Pump 1 can empty a pool in 9 hours while Pump 2 can empty the same pool in 12 hours. If the two pumps worked together, how long would it take them to empty the pool?

about 5 hours and 30 minutes
about 5 hours and 10 minutes
about 5 hours and 20 minutes
about 5 hours

Answer 1

About 5 hr 10 min

Explanation:

rate 1 = 1 pool / 9 hrs rate 2 = 1 pool / 12 hours

now find the time with both rates combined

1 pool / ( 1/9 + 1/12) = 1/ (7/36) = 36/7 hr = 5 1/7 hr = 5 hr 8.6 min