Question

diesel engines use about 1 gallon of fuel per hour for every 18 horsepower used. How many hours can the boat run at 342 horsepower on a half tank of fuel?

Answer 1

For this problem we are going to use a 400 gallons tank. You can apply the same procedures to a bigger or smaller tank.

The first thing we are going to do id divide the horsepower of the engine (342) by the rate of gallon consumption (18) to find how many gallons our engine will consume in one hour: 
$\frac{342}{18} =19$ gallons per hour.

Next, we are going to divide the half tank of fuel by our engine consumption per hour:
Since the full tank is 400 gallons, half tank will be 200 gallons.
$\frac{200}{19} =10.5$ hours 

We can conclude that the boat will run for 10.5 hours at 342 horsepower on a half tank of fuel.