3 pumps, working 6 hours a day, can empy a water tank in 2 days. How many hours a day must 9 pumps work to empty the same tank in 1 day?

Answer 1

3 pumps ----> working 6 hours --- 2 days
6 pumps ----> working 6 hours --- 1 day
1 pump -----> working 36 hrs ---- -1 day
9 pumps ----> working 4 hrs ------ 1 day

or

it takes 18 pump-hrs running for 2 days
it takes 36 pump-hrs to do it in 1 day
so 9 pumps would take 4 hrs to do it in 1 day