How old is the 11th person?

Math Central

Quandaries & Queries

Question from Jamie, a student:

Ten people are in a room, and their mean age is 15. After another person enters the room, the mean age is now 18. How old is the person that walked in?

Hi Jamie.

Knowing the mean age and the number of people, you can find the total of their ages.

So if you know that the original ten had a mean age of 15, then the total of their ages was 150.

Later, there are 11 people in the room and the mean age is 18. So what is the total of the ages now?

The only difference is in the one person that walked in, so what must her age be?

Hope this helps you,
Jesse McLeod and Stephen La Rocque.

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