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Algebra Difficulty 1.5 Junior Find the answer Canada

The average (mean) of a list of $10$ numbers is $17$ . When one number is removed from the
list, the new average is $16$ . What
number was removed?

Pick one

Solution

Since $385$ has a units digit
of $5$ , then it is divisible by $5$ . Dividing, we get $\frac{385}{5}=77$ .

Since $77=7\times11$ , then the three
prime factors of $385$ are $5$ , $7$
and $11$ , and their sum is $5+7+11=23$ .

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.