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Riding Rookies

Solution

There are two 8-year-olds, two 10-year-olds, and four 11-year-olds. (They are all a year older.)

Explanation

Last year the distribution looked like this.
7  7  9  9  10  10  10  10

Ten was the mode (the age that occurred most often). There were two 7-year-olds and two 9-year-olds. The mean age was 9 (72 divided by 8).

This year the distribution looks like this.
8  8  10  10  11  11  11  11