MathWorld is a little online math encyclopedia.
It has been popular since the late nineties.
Learn math in little bits by clicking around or by entering search terms at MathWorld.
For example, here is the entry on the Arithmetic Mean  the commonest "average."
Nifty, eh?
Now try to answer this question applying the arithmetic mean:
Five students in a class have an average age of 20 years.
Of the five, three of them have the same age.
Fill in the list of their ages: 18, 21, x , y , z
Note: only whole numbers are allowed.
This is really not fair

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In case you want the answer, there are two correct answers:
ReplyDeletetwo more 18s, with 25 to complete the set;
or two more 21s, with a 19 year old.
There are no other correct answers.