Hello!
If we want to find the mean (average) of any set of numbers, all you need to do is add the set together, then divide by how many numbers are in the set.
When we add all the numbers we get: 100
Next we just divide by the number of people which is 10: 100 divided by 10 is 10
So, the mean age of the swimmers on the team is an even number of 10.