2.1. (5 points) One side of a rectangle is $20 \%$ smaller than the other, and its area is $4 \frac{1}{20}$. Find the larger side of the rectangle.
Answer: 2.25.

Solution. If $a-$ is the side we are looking for, then from the condition of the problem $0.8 a^{2}=\frac{81}{20}$, from which $a^{2}=\frac{81}{16}, a=\frac{9}{4}$.