I5.1 Two opposite sides of a rectangle are increased by $50 \%$ while the other two are decreased by $20 \%$. If the area of the rectangle is increased by $a \%$, find $a$.
Let the length and width be $x$ and $y$ respectively.
$$
\begin{array}{l}
1.5 x \times 0.8 y=1.2 x y \\
\Rightarrow a=20
\end{array}
$$