Let's assume that it takes x hours to travel from City A to City B. Since the boat travels at an average speed of 10 miles per hour, the distance from City A to City B is 10x miles.
Similarly, the distance from City B to City C is 15(2x) = 30x miles, as the boat travels at an average speed of 15 miles per hour for 2x hours.
The total distance traveled is 10x + 30x = 40x miles, which is equal to 150 miles.
Therefore, 40x = 150.
Simplifying the equation, we get x = 150/40 = 3.75.
Since it takes x hours to travel from City A to City B, the entire trip takes 3.75 + 3.75 = <<3.75+3.75=7.50>>7.50 hours. Answer: \boxed{7.50}.