Bunuel
An ice-cream sundae consists of two ice-cream scoops, one flavor per scoop, and one topping. How many different types of sundaes can be prepared if four ice-cream flavors and two toppings are available?
(A) 12
(B) 14
(C) 16
(D) 18
(E) 20
When answer choices are low in a Combinatorics question, I'll usually diagram out the options, as it's safer and spares me the trouble of worrying about which formula to use, what to divide it by, etc. In this case, I'd call the four ice-cream flavors A, B, C, and D and proceed to diagram the options for the two scoops as follows (noting that the problem doesn't explicitly require us to use two different flavors for the sundae):
AA
AB
AC
AD
BB
BC
BD
CC
CD
DD
So there are 10 possibilities for the flavors of the two scoops in the sundae. Now there are toppings to consider. For each of the 10 flavor combinations listed above, there are two options for toppings. This means that there are 10 scoop combinations with topping 1 and the same 10 scoop combinations with topping 2, for a total of 10 + 10 = 20 combinations. The correct answer is (E).