Explanation:
if one pump = x
the other pump = 2x
therefore
x + 2x = 8 minutes
3x = 8 minutes
Divide through by 3
x = 8/3 minutes
converting it to seconds
x = 8/3 * 60
x = 160 seconds
therefore
the first pump = 2x = 2 * 160 = 320 seconds
the second pump = x = 160 seconds