3. A rectangle is divided into four smaller rectangles by two straight lines. The areas of three of these rectangles are 20 mu, 25 mu, and 30 mu. What is the area of the other rectangle in mu?
3. 【Solution】Let the area of the rectangle be 25 mu, with length $\mathrm{a}$ and width $\mathrm{b}$; the area of the rectangle be 30 mu, with length $\mathrm{c}$ and width $\mathrm{d}$; then the area of the rectangle be 20 mu, with length $\mathrm{c}$ and width $\mathrm{b}$; and the area of the rectangle to be found, with length $a$ and width $d$, is
$$
\mathrm{a} \times \mathrm{d}=\frac{(a \times b) \times(c \times d)}{b \times c}=\frac{25 \times 30}{20}=37.5 \text { (mu) }
$$