Henry's Hamburger Haven offers its hamburgers with the following condiments: ketchup, mustard, mayonnaise, tomato, lettuce, pickles, cheese, and onions. A customer can choose one, two,or three meat patties and any collection of condiments. How many different kinds of hamburgers can be ordered?
- A)
- B)
- C)
- D)
- E)
Answer
C
Key insight
Each of the 8 condiments is independently in or out, giving 2^8 subsets, times 3 patty choices.
Solution
A hamburger is specified by two independent decisions.
Condiments: for each of the condiments the customer decides yes or no, so there are possible collections (including the empty one).
Patties: options.
By the multiplication principle the number of hamburgers is .
The answer is .
Why this works
"Any collection of" a set of items means any subset, and a set with elements has subsets because each element is independently included or excluded. Independent choices multiply. Distinguish this from arranging the condiments in order, which would be a factorial and is not what is being asked.
The trap
Using 8! = 40320 for the condiments as if their order mattered, choice (D).
Common mistakes
- Using 8! = 40320 for the condiments as if their order mattered, choice (D).
- Adding or forgetting the "no condiments" option (giving ).
Techniques
Set up the equation/formula and compute; no special trick needed