You appear to have answered 10 and 12 correctly.
11. Since the fee charged is more than $10, we know the boat was out more than 3 hours. It is $8 more than that amount, so was out for an additional 8*15 minutes = 120 minutes = 2 hours.
Bernice and Ernesto kept the paddle boat for 3 + 2 = 5 hours.
