21. A plot of land in the shape of a rectangle was allocated for corn. After some time, the length of this plot was increased by $35\%$, and the width was decreased by $14\%$. By what percentage did the area of the plot change?
21. Let the length of the plot be $a$, and the width be $b$, then its area is $a b$. After increasing the length by $35\%$, it became $1.35 a$, and after decreasing the width, it became $0.86 b$. The area of the plot became: $1.35 a \times 0.86 b = 1.161 a b$. Therefore, the area increased by $1.161 a b - a b = 0.161 a b$, which is $\frac{0.161 a b}{a b} \cdot 100\% = 16.1\%$.