A relay team of 4 swimmers is to be formed from a swim team with 20 member. How many different relay teams are possible?

When order does not matter you use what is called n choose k formula

n! / (k! (n-k) !) where n is the number of things to choose from and k is the number of choices made, in this case:

20! / (4! (20-4) !) = 4845