Question 1163560
<br>
Another quick mental solution method -- very similar to, yet much different than, the one shown by tutor @ikleyn.<br>
In 12 hours, the new pump could drain the pool 2 times -- or, more reasonably, it could drain 2 of the pools.
In the same 12 hours, the old pump can drain the pool once.
So together in 12 hours the two pumps could drain 3 of the pools.
And therefore the two together can drain the one pool in 12/3 = 4 hours.<br>
That's the same logic as used in tutor @ikleyn's mental solution; but the thought process is a bit different.<br>
Try both methods to see if one or the other "works" better for you.<br>