4. (15 points) During a walk, a pedestrian first walked $3 \mathrm{km}$ at a speed of $1.5 \mathrm{m} / \mathrm{s}$, and then another $3600 \mathrm{m}$ at a speed of $3.6 \kappa \mathrm{m} / \mathrm{h}$. Determine his average speed for the entire walk.
Answer: $\approx 1.18 \frac{M}{s}$

Solution. Time spent on the first part of the journey $t_{1}=\frac{s_{1}}{v_{1}}=\frac{3000}{1.5}=2000$ s. Time spent on the second part of the journey $t_{2}=\frac{s_{2}}{v_{2}}=\frac{3600}{1}=3600$ s. Average speed for the entire walk: $v_{av}=\frac{s_{1}+s_{2}}{t_{1}+t_{2}}=\frac{3000+3600}{2000+3600} \approx 1.18 \frac{M}{s}$