Let us start with the time it takes for 1 gallon of gas to be dispensed. If ½ gallon takes 3 ¼ seconds, it will take double of that time for one gallon of gas. Double of 3 ¼ seconds is 6 1/8 seconds. Unit rate is 1 gallon of gas at every 6 1/8 seconds If one gallon of gas takes 6 1/8 seconds, 50 gallons will take 50 x 6 1/8 seconds. That is, 325 seconds or 5 minutes ans and 25 seconds.
