The "per gallon" means "ONE gallon"
Divide both parts of the ratio 50 miles: 2.5 gallons by 2.5
50/2.5 = 20
2.5/2.5 = 1
So the ratio 50 miles:2.5 gallons reduces to 20 miles:1 gallon.
Answer: The unit rate is 20 miles per gallon. One gallon of gas will allow you to drive 20 miles.