This is a classic trick question.
It involves the dangerous practice of averaging averages.
I once had to explain to a Dean at my college who did this.
I wrote him a long rant and finally got through to him.
Suppose administrators are evaluted by the Faculty.
To be retained, he must have the approval of 80% of the Faculty.
At our college, there are 100 professors of which two are on the math faculty.
. . Everyone approved of the Dean except one math professor.
Should the Dean be retained?
According to his math, the answer is a clear NO.
In the math department, he received only 50% approval.
In the rest of the faculty, he received 100% approval.
The average of 50% and 100% is: .(50% + 100%)/2 .= .75%.
. . Hence, it is with some regret that the Dean must be dismissed.
"But," he will argue, "if 99 out of 100 approved me, don't I have 99% approval?"
"Sorry," I would reply, "the math is clear and undeniable: only 75%.
. . Please close the door on your way out."