With both valves open

Hey you need to do a small correction. I hope I am correct,because that is the only way I figured out this question can be solved.

What is the capacity of the pool if every minute the second valve admits 50 cubic meters of water more than the first?

I think this would be reverse : if every minute the first valve admits 50 cubic meters of water more than the second?
IMO 12000

A(1st valve) can fill in 2 hrs
together = 48 mints =4/5 hrs

1/a + 1/b = 5/4

1/2 +1/b = 5/4

b= 4 hrs
b= second valve
Now its given
every minute the first valve admits 50 cubic meters of water more than the second

Since A can fill in 2 hrs,so definitely speed of A is higher than that of B

If B has speed of V
Speed of A =2V

Now look at what's the condition

2v = v+50

v= 50

capacity of pool = speed of B* time taken by B
= 50*4hrs*60(speed is given per minute)
=12000

Hope this helps

GMAT740: Thanks for your explanation!

there is no typo.