How to solve it because i have no idea

Asequence is defined recursively using the formula f(n + 1) =f(n) - 5. which sequence could be

Afamily has two cars. the first car has a fuel efficiency of 25 miles per gallon of gas and the second has a fuel efficiency of 15 miles per gallon of gas. during one particular week, the two cars went a combined total of 1025 miles, for a total gas consumption of 55 gallons. how many gallons were consumed by each of the two cars that week?

Ineed with number 8 and answer fast because i only have 11 minutes or else i get a f for my report card