If A and B are two sets such that n(a)=150 , n(b)=250 and n(A U B)=300.

Find n(A - B) and n(B - A).

According to the given data we can show it via Venn diagram as, 

Assuming that there is no element other than A and B ,
Now it is given that , 
n(A) = 150 , n(B) = 250 and n(A$\cup$B) = 300 so, using the formula, 

$n(A\cup B) =$n(A)+​n(B)- n (A$\cap$B)
300 = 150 + 250 -​ n (A$\cap$B)
Or ​ n (A$\cap$B) = 100
And now we know that , 
n(A-B) = $\left\{x : x\in A and x \mathrm{does} \mathrm{not} \mathrm{belong} \mathrm{to} B\right\}$
So ​n(A-B) = ​n(A)-​n (A$\cap$B) = 150-100 = 50
Similarly , 
 n(B-A) = n(B)-n (A$\cap$B)
=250-100 = 150
Answer