\begin{aligned}
& P \alpha \sqrt{Q} \\
& \Rightarrow P=K \sqrt{Q} \text { where } K \text { is constant } \\
K &=\frac{p}{\sqrt{Q}} \\
& \Rightarrow \frac{P_{1}}{\sqrt{Q_{1}}}=\frac{P_{2}}{\sqrt{Q_{2}}} \\
\text { If } P &=20=P_{1} \cdot Q=4=Q_{1} \\
Q_{2} &=? P_{2}=100 \text { so that } * \text { becomes } \\
\frac{20}{\sqrt{4}} &=\frac{100}{\sqrt{Q_{2}}}: \cdot \frac{20}{2}=\frac{100}{\sqrt{Q_{2}}} \\
\sqrt{Q_{2}} \times 20 &=2 \times 100 \\
\sqrt{Q_{2}} &=\frac{2 \times 100}{20}=10: \\
\sqrt{Q_{2}} &=10 \\
\text { square both sides }
\end{aligned}
\(Q_{2}=10^{2}=100\)