A restaurant 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