Your mission is to make a miles per gallon computer.

* Asks how far they traveled.
* Asks how much gas they used.
* The amount of gas must be positive and less than 100 gallons.
* The distance travelled must be non-negative and less than 3,000 miles.
* If anything is bad, turn it red and don't compute the answer.
* Put happy picture if they get more than 30 mpg, and a sad picture otherwise.

Useful example at http://cs.nmu.edu/~rappleto/Classes/CS101/cookies.html