Each person needs 2 mittens. Since you have 18 mittens (18 /2) will be the number of of people who get mittens.
Therefore each person will also get 1 winter hat leaving 3 that can be saved for next year.
This problem looks trivial. It is not. Many chemistry problems ask a question such that one chemical is in greater supply than another in a reaction. There is some of one of them left over. Same as the mittens. It's good to think in terms of leftovers.