A 3 3−8
-ounce scoop of strawberry ice cream
was stacked on top of a 2
7−8
-ounce scoop of
chocolate ice cream. What is the total
weight of ice cream on the cone?

There are (63) combinations. The notation means "six choose three". Out of six items (flavors) choose three.

(nk)=n!k!(n−k)!.

(63)=6!3!3!.

Think of it this way. There are 6 ways to choose a flavor. Once you choose, there are 5 ways to choose the next. After that, there are 4 flavors left. which is 6!/3!=6⋅5⋅4⋅3⋅2⋅13⋅2⋅1=6⋅5⋅4=120.

But, you could have chosen {chocolate,vanilla,strawberry} and you get the same combination as {vanilla, strawberry, chocolate} so we have to divide by 3!=3⋅2⋅1=6 to account for the order of choosing.

So the number of combinations of flavors is (63)=1206=20.