A large pile of coins consists of pennies nickels dimes and

A large pile of coins consists of pennies, nickels, dimes, and quarters (at least 20 of each). How many different collections of 20 coins can be chosen? DO NOT USE COMMAS IN YOUR ANSWER!

C(23,3) P.S. I know the answer but no clue what professor is trying to say by no commas....

Solution

There are 4 ways of choosing each coin.

So the required answer is:

4^20

=109511627776