An empty pool being filled with water at a constant rate takes 8 hours to fill to 3/5 of its capacity. How much more time will it take to finish filing the pool?

My Work:

Let t = time (in hours)

1 - 3/5 = 2/5

8/t = (3/5)/(2/5)

Solving for t, I got 5.3333333333, which of course I rounded to two decimal places for a final answer of 5 hours and 30 minutes.

The book's answer is 5 hours and 20 minutes.

Why am I wrong? Is my proportion set up incorrect?