Alright! In order to find the answer you first have to find out how many miles you can go on one gallon of gas. To do this you divide your miles by your gallons:
[400 ÷ 15] = 26.6
Then you take this value and multiply it by 6.
[26.6 x 6] = 160.
You can go 160 miles on 6 gallons of gas.