What you need to do is find out how many miles he got per gallon. Do this by dividing 420 by 18. You get 23.33333, so about 23 miles per gallon. So now that you know you get 23 miles per gallon, divide 357 by 23 to find how many gallons. You get 15.52. So I would round it up to 16. Does this help? Do you need more clarification?