7.2. The two adjacent sides of a rectangle are in the ratio 3:7. What is the area of the rectangle if its perimeter is 40 cm?
Answer: $84 \mathrm{~cm}^{2}$. Solution. Let $x$ be the shorter side of the rectangle, then $\frac{7}{3} x$ is the longer side. From the conditions of the problem, we get the equation $2\left(x+\frac{7}{3} x\right)=40$. Hence, $x=6$ (cm), and the area is $x \frac{7}{3} x==84$ $\left(\mathrm{cm}^{2}\right)$.