Question

If a sundae must have exactly 4 toppings, how many different sundaes can you make?

Answer 1

The complete question in the attached figure

we know that
the formula for combinations is
C=n!/[(n-r)!*r!]
where
n is the total number of objets
r is the number that you choose to arrange 

in this problem
n=6
r=4
C=6!/[(6-4)!*4!]-----> C=(6*5)*4!/[2!*4!]----> C=30/2----> C=15

the answer is
15