4. (15 points) During a walk, a pedestrian first walked $2.8 \kappa m$ at a speed of $1.4 m / s$, and then ran another 1800 m at a speed of 7.2 km/h. Determine his average speed for the entire walk.
Answer: $\approx 1.59 \frac{M}{c}$

Solution. Time spent on the first part of the journey $t_{1}=\frac{S_{1}}{v_{1}}=\frac{2800}{1.4}=2000 \mathrm{c}$. Time spent on the second part of the journey $t_{2}=\frac{s_{2}}{v_{2}}=\frac{1800}{2}=900 \mathrm{c}$. Average speed for the entire walk: $v_{c p}=\frac{s_{1}+s_{2}}{t_{1}+t_{2}}=\frac{2800+1800}{2000+900} \approx 1.59 \frac{\mathrm{M}}{c}$