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Review: Comparing Two Averages
Explanation
This question can be solved in various different ways that
all boil down to the average formula. The average of the first three numbers is
The average of the last three numbers is 10 higher, so
it's 85. The differences of the three numbers from 85 must sum to zero. We have
and 
so the net so far is -30, so the last number
must be 
.
The correct answer is therefore (B).
Note: if you hadn't known about or didn't want to use the
"differences" technique, you could have re-formed the average formula as 
and solved for x, but I would contend that that method is slower for roughly 100%
of people.
Again, the correct answer is (B).
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