In a whole hour, a half-hour goes by, two times.
In the first half-hour, 7/8 of a gallon leaks out of the pool.
In the second half-hour, another 7/8 of a gallon leaks out of the pool.
In the whole hour, (7/8 gallon + 7/8 gallon) leaks from the pool.
Can you add those two fractions, and make a single number ?