Question

if there are 7 teams and every teams plays everyone once how many games total played

Answer 1

This is a problem about combinations where the order doesn't matter. The solution is usually written as 7C2 (seven choose two) and has the value

$\frac{7!}{(7-2)!2!}=21$

Comment: 7C2 is the answer to the question "How many pairs (in our case, these pairs are seen as games played) can we form from a group of 7 things?".