3. A rectangular photograph with dimensions of 30 cm and 4 dm was enlarged several times to create a rectangular billboard. The area of the billboard is 48 m². What are its length and width?
3. 4) $30 \times 40=1200\left(\mathrm{~cm}^{2}\right)$ - the area of the photograph;
2) $480000: 1200=400$ (times) - the area of the billboard is 400 times larger than the area of the photograph.

Since when each side of a rectangle is increased by $k$ times, its area increases by $k \times k$ times, in our case, each side increases by 20 times $(20 \times 20=400)$
3) $30 \times 20=600$ (cm) - the width of the billboard;
4) $40 \times 20=800$ (cm) - the length of the billboard.

Answer: the width of the billboard is 6 m, the length is 8 m.