20 people at a party gives gifts to each other. How many gifts were given?
20 people at a party shake each others hands. How many handshakes were there?
An ice cream store has 20 flavors. How many different ways can you pick 2 different flavors?
When Gauss was very young he found a fast way to add up all the numbers from 1 to 100. Can you?
What is the nth triangle number?