Joe goes to a mexican restaurant and order nachos. He can have just cheese or add na of the following: chicken, onion, sour cream or olives. How may different variations of nachos are possible

1) cheese and chicken 2) cheese and onion 3) cheese and sour cream 4) cheese and olives 5) cheese, chicken, and onion. 6) cheese chicken and sour cream 7) cheese, chicken, and olives 8) cheese, sour cream, and olives 9) cheese 10) chicken and oinion 11) chicken and sour cream 12) chicken and olives 13) onion and sour cream 14) onion and olives 15) sour cream and olives