If the first valve pumps x cubic feet of gas per minute, then the two valves together will pump x + x + 20 cubic feet of gas per minute. At the first rate, x , the tank reaches capacity in 120 minutes; at the second, 2x + 20, it takes only 50 minutes. So:
Capacity = x * 120 = (x + x + 20) * 50
We can solve for the rate of the first valve:
120x = 50 * 2x + 1000
20x = 1000
x = 50
Since the lone valve fills in 120 minutes, the capacity is 120 * 50 = 6000.
The correct answer is (B).