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1.Set Theory
easy
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?
A
$30$
B
$30$
C
$30$
D
$30$
Solution
It is given that:
$n(X)=40, n(X \cup Y)=60, n(X \cap Y)=10$
We know that:
$n(X \cup Y)=n(X)+n(Y)-n(X \cap Y)$
$\therefore 60=40+n(Y)-10$
$\therefore n(Y)=60-(40-10)=30$
Thus, the set $Y$ has $30$ elements.
Standard 11
Mathematics
