Hi K.
so if we look at the amount of gas burned each hour
we have 5, 10, 20, 40, 80, ....
we have a sequence with a common multiple of 2
we can use the formula to find the sum but in this case we can solve by addition
after four hours the boat will have used 75 gallons 5 + 10 + 20 + 40
after five hours the boat will have used 155 gallons 75 + 80
so 25/80 * 60 minutes = 18.75 minutes
so after 4 hours and 18.75 minutes the boats runs out of gas