Help i will give u brainlest Erin buys gas at a gas station for $4 a gallon. There is a sale going on at the gas station so that anyone who buys more than 5 gallons of gas only has to pay $3 a gallon for each gallon over 5 gallons. If a person buys more than 10 gallons of gas, each gallon over 10 gallons is only $2 a gallon for every gallon over 10 gallons but less than or equal to 15 gallons.. Erin's tank holds 12 gallons of gas.