Let X be the employed students..
n( E only ) + n( S only ) + X = 42
[ 29 – X ] + [ 24 – X ] + X = 42
X = 11
I find some gaps in your fomula
first: I think the correct is "-" not "+", if "+" X can not be 11
second: from the information of the problems, you can not assume that the group of none-student and none-employed is zero.
That is why answer should be E
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