What does mutually exclusive mean?
It means that one thing can not happen together with another thing.
It's always an EITHER...OR thing, but never both.
If you flip a coin, you get EITHER heads OR tails.
If you select 4 different numbers out of 10, and you've selected your first number, you cannot select that number again.
In formula: 1st number 10 choices, 2nd number 9 choices, etc., or
Choosing 4 out of 10:
(if you could choose the same number again it would be