Problem 4. A farmer's field has the shape of a rectangle, with a length that is twice the width. It is fenced with three strands of wire, for which $360 \mathrm{~m}$ of wire was used. The field was sown with wheat. How many kilograms of wheat were harvested, if $35 \mathrm{~kg}$ of wheat is obtained from every $100 \mathrm{~m}^{2}$?
Solution. The dimensions of the field are width $x$ and length $2 x$. The perimeter of the field is

$$
L=2(x+2 x)=6 x
$$

If the field is enclosed with three rows of wire with a total length of $360 \mathrm{~m}$, then for one row of wire, $360: 3=120$ meters of wire are needed, which is also the perimeter of the field. Clearly, then $6 x=120 \mathrm{~m}$, so the width of the field is $x=20 \mathrm{~m}$, and the length is $40 \mathrm{~m}$. The area of the field is

$$
P=20 \cdot 40 \mathrm{~m}^{2}=800 \mathrm{~m}^{2}
$$

The total amount of wheat harvested is

$$
8 \cdot 35 \mathrm{~kg}=280 \mathrm{~kg}
$$