\(U=100, n(1)=20, n\left(Y^{\prime}\right)=20 \text {, }\)
\(n(1 \cup Y)^{\prime}=10\)
\begin{array}{l}
n\left(Y^{\prime}\right)=n\left(I \cap Y^{\prime}\right)+n(I \cup Y)^{\prime} \\
20=20-x+10 \\
20+30-x, x=30-20=10 \\
\end{array}