$$\begin{gathered} If n(P-Q)=x+37 , n(Q-P)=30+3x, n(P\cup Q)=120+2x \\ and n(P\cap Q)=35, then x= \end{gathered}$$

$$\begin{gathered} n(P-Q)=x+37 , n(P\cup Q)=120+2x \\ n(Q-P)=30+3x, n(P\cap Q)=35 \\ We have n(P\cup Q)=n(P-Q)+n(Q-P)+n(P\cap Q) \\ \Rightarrow 120+2x=x+37+30+3x+35 \\ \Rightarrow 120+2x=4x+102 \\ \Rightarrow x=9 \end{gathered}$$