If you would like to know how long will it take to drain the pool, you can calculate this using the following steps:
3 hours ... 250 gallons
x hours = ? ... 150 gallons (= 400 - 250)
3 * 150 = 250 * x
x = 3 * 150 / 250
x = 1.8 hours
3 hours + 1.8 hours = 4.8 hours
The correct result would be B. 4.8 hours.
Let's say the numbers are "a" and "b"
now, we know one of them is 7 more than the other, so hmmm let's say "b" is 7 more than "a", so b = a + 7
the difference of their squares is 161, alrite

and surey you know how much that is
now, what's "b"? well, b = a + 7