Motorcyclist Cyclist Speeds 9E776D
1. **Problem statement:** A motorcyclist and a cyclist start from points A and B, 180 km apart, and ride towards each other. They meet after 2 o'clock and continue without stopping. The motorcycle reaches B 3 hours earlier than the cyclist reaches A. We need to find their speeds.
2. **Define variables:** Let $v_m$ be the speed of the motorcyclist (km/h) and $v_c$ be the speed of the cyclist (km/h).
3. **Time relations:** Let $t$ be the time (in hours) from 2 o'clock to the meeting point.
- Distance covered by motorcyclist until meeting: $d_m = v_m t$
- Distance covered by cyclist until meeting: $d_c = v_c t$
Since they start at the same time and meet after 2 o'clock, the sum of distances is total distance:
$$d_m + d_c = 180$$
$$v_m t + v_c t = 180$$
$$t(v_m + v_c) = 180$$
$$t = \frac{180}{v_m + v_c}$$
4. **Time to reach destinations after meeting:**
- Motorcyclist remaining distance: $180 - d_m = 180 - v_m t$
- Cyclist remaining distance: $180 - d_c = 180 - v_c t$
Time taken after meeting:
- Motorcyclist: $\frac{180 - v_m t}{v_m}$
- Cyclist: $\frac{180 - v_c t}{v_c}$
5. **Given condition:** Motorcyclist reaches B 3 hours earlier than cyclist reaches A:
$$\frac{180 - v_c t}{v_c} - \frac{180 - v_m t}{v_m} = 3$$
6. **Substitute $t$ from step 3:**
$$\frac{180 - v_c \frac{180}{v_m + v_c}}{v_c} - \frac{180 - v_m \frac{180}{v_m + v_c}}{v_m} = 3$$
Simplify numerator terms:
$$\frac{180 - \frac{180 v_c}{v_m + v_c}}{v_c} - \frac{180 - \frac{180 v_m}{v_m + v_c}}{v_m} = 3$$
$$\frac{180 \left(1 - \frac{v_c}{v_m + v_c}\right)}{v_c} - \frac{180 \left(1 - \frac{v_m}{v_m + v_c}\right)}{v_m} = 3$$
Simplify inside parentheses:
$$\frac{180 \frac{v_m}{v_m + v_c}}{v_c} - \frac{180 \frac{v_c}{v_m + v_c}}{v_m} = 3$$
Rewrite:
$$\frac{180 v_m}{v_c (v_m + v_c)} - \frac{180 v_c}{v_m (v_m + v_c)} = 3$$
Multiply both sides by $v_m v_c (v_m + v_c)$:
$$180 v_m^2 - 180 v_c^2 = 3 v_m v_c (v_m + v_c)$$
Divide both sides by 3:
$$60 v_m^2 - 60 v_c^2 = v_m v_c (v_m + v_c)$$
7. **Rewrite equation:**
$$60 (v_m^2 - v_c^2) = v_m v_c (v_m + v_c)$$
Note that $v_m^2 - v_c^2 = (v_m - v_c)(v_m + v_c)$, so:
$$60 (v_m - v_c)(v_m + v_c) = v_m v_c (v_m + v_c)$$
Divide both sides by $(v_m + v_c)$ (nonzero speeds):
$$60 (v_m - v_c) = v_m v_c$$
8. **System of equations:**
- From step 3: $$t = \frac{180}{v_m + v_c}$$
- From step 7: $$60 (v_m - v_c) = v_m v_c$$
9. **Express $v_m$ in terms of $v_c$:**
$$60 (v_m - v_c) = v_m v_c$$
$$60 v_m - 60 v_c = v_m v_c$$
$$60 v_m - v_m v_c = 60 v_c$$
$$v_m (60 - v_c) = 60 v_c$$
$$v_m = \frac{60 v_c}{60 - v_c}$$
10. **Substitute $v_m$ into total distance equation:**
$$t = \frac{180}{v_m + v_c} = \frac{180}{\frac{60 v_c}{60 - v_c} + v_c} = \frac{180}{\frac{60 v_c + v_c (60 - v_c)}{60 - v_c}} = \frac{180 (60 - v_c)}{60 v_c + 60 v_c - v_c^2} = \frac{180 (60 - v_c)}{120 v_c - v_c^2}$$
11. **Time to meet must be positive and less than total travel time. Try values for $v_c$ between 0 and 60:**
Try $v_c = 20$:
$$v_m = \frac{60 \times 20}{60 - 20} = \frac{1200}{40} = 30$$
Check equation 7:
$$60 (30 - 20) = 60 \times 10 = 600$$
$$v_m v_c = 30 \times 20 = 600$$
True.
12. **Calculate meeting time:**
$$t = \frac{180}{30 + 20} = \frac{180}{50} = 3.6 \text{ hours}$$
13. **Calculate times after meeting:**
- Motorcyclist remaining distance: $180 - 30 \times 3.6 = 180 - 108 = 72$ km
- Time for motorcyclist after meeting: $\frac{72}{30} = 2.4$ hours
- Cyclist remaining distance: $180 - 20 \times 3.6 = 180 - 72 = 108$ km
- Time for cyclist after meeting: $\frac{108}{20} = 5.4$ hours
Difference: $5.4 - 2.4 = 3$ hours, matches the problem.
14. **Final speeds:**
- Motorcyclist speed: 30 km/h
- Cyclist speed: 20 km/h
15. **Table:**
| Vehicle | Speed (km/h) | Distance to Meeting (km) | Time to Meeting (h) | Remaining Distance (km) | Time After Meeting (h) |
|--------------|--------------|-------------------------|---------------------|------------------------|-----------------------|
| Motorcyclist | 30 | 108 | 3.6 | 72 | 2.4 |
| Cyclist | 20 | 72 | 3.6 | 108 | 5.4 |