If the diameter is 16 inches, then its radius is half that, r = 8.
(1/12)X = 6800
6800/ (1/12) = X
X = 81600 (hours)
6800 (6.8 * 10^3) divided by 1/12 (12^-1) equals 81600. It will take 81600 hours for the pool to completely empty at a rate of 0.833333333 or 12^-1 gallons per second.
ANSWER

EXPLANATION
The given fractions are:

We factor to obtain:

We cancel the common factors to get:

We multiply the numerators and also multiply the denominators to get:

Therefore the two fractions simplifies to 