An ice cream store offers 20 different topping combinations, each one composed of two different items. Determine how many actual topping there are.

So i'm not sure at all how to do this question but what i think is you have to divide the 20 by 2 getting 10 and then do 10nPr2 which equals 90. I know this is not right at all but i really am lost here.

The answer in the back of the book is 5

Would i just divided the 10 by 2 again? Why? I don't understand (Worried)