Lab #1

Figure out how much gas is needed to fill up a car.

Fact's about the car:
	It can hold 20 gallons of gas.

Ask the person to enter in the amount of gas in the car (in whole amounts of gallons).
Display the amount of gas needed to fill up the car to a full tank of gas.

For example if the person says there is 5 gallons of gas in the car,
they can fill up the tank with 15 gallons of gas.

Please write a comment at the begining of the program explaining
what the purpose of the program is.