Arestaurant has 5 pizza toppings to choose from. how many different 2-topping pizzas are possible?

10

Because if you do 5•2 you get 10

More in depth explanation:

Though there are 25 possible configurations the question asks for two different toppings together. It also asks for unique combinations. So AB and BA are the same combination in this context. The only unique possibilities are

AB
AC
AD
AE
BC
BD
BE
CD
CE
DE

It is easy to simplify this into 5•2 for this situation. And if the question asked for three toppings you would do 5•3.

However if the question asked for the configurations for two toppings then you would do 5•5 and if it asked for the configurations of 3 toppings you would do 5•5•5

These are the multiples of 15 but what you are probably asking or what are the factors of 15