If X and Y are two sets such that X has 40 elements,`X cup Y` has 60 elements and `X cap Y` has 10 elements, how many elements does Y have?

Explanation

`n(X)=40, n(X uu Y)=60, n(X nn Y)=10`

`n(X uu Y)=n(X)+n(Y)-n(X nn Y)`

60=40+n(Y)-10

n(Y)=60-40+10=30
therefore Y has 30 elements.
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