A model car is tested on some closed circuit $600$ meters long, consisting of flat stretches, uphill and downhill. All uphill and downhill have the same slope. The test highlights the following facts:

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(a) The velocity of the car depends only on the fact that the car is driving along a stretch of uphill, plane or downhill; calling these three velocities $v_s, v_p$ and $v_d$ respectively, we have $v_s <v_p <v_d$;

(b) $v_s,v_p$ and $v_d$, expressed in meter per second, are integers.

(c) Whatever may be the structure of the circuit, the time taken to complete the circuit is always $50$ seconds.
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Find all possible values of $v_s, v_p$ and $v_d$.
1. **Define Variables and Equations:**
   Let the total length of the circuit be \(600\) meters. Let \(s\) be the length of the uphill track, \(p\) be the length of the plane track, and \(d\) be the length of the downhill track. Since the circuit is closed, the total length is given by:
   \[
   s + p + d = 600
   \]
   Given that the circuit is symmetric in terms of uphill and downhill, we have \(s = d\). Therefore, the equation simplifies to:
   \[
   2d + p = 600
   \]

2. **Time Equation:**
   The time taken to complete the circuit is always \(50\) seconds. The time taken to travel each segment is given by the distance divided by the velocity. Let \(v_s\) be the velocity on the uphill, \(v_p\) be the velocity on the plane, and \(v_d\) be the velocity on the downhill. The total time equation is:
   \[
   \frac{d}{v_s} + \frac{p}{v_p} + \frac{d}{v_d} = 50
   \]

3. **Substitute \(p\):**
   From the length equation, we have \(p = 600 - 2d\). Substitute this into the time equation:
   \[
   \frac{d}{v_s} + \frac{600 - 2d}{v_p} + \frac{d}{v_d} = 50
   \]

4. **Combine Terms:**
   Combine the terms involving \(d\):
   \[
   d \left( \frac{1}{v_s} + \frac{1}{v_d} \right) + \frac{600 - 2d}{v_p} = 50
   \]

5. **Isolate \(d\):**
   Rearrange to isolate \(d\):
   \[
   d \left( \frac{1}{v_s} + \frac{1}{v_d} - \frac{2}{v_p} \right) = 50 - \frac{600}{v_p}
   \]

6. **Simplify:**
   Since \(v_s < v_p < v_d\) and all velocities are integers, we need to find values of \(v_s\), \(v_p\), and \(v_d\) that satisfy the equation. We also know that \(v_p\) must be a divisor of \(600\) to make the equation work with integers.

7. **Check Possible Values:**
   Let's test possible values for \(v_p\). Since \(v_p\) must be an integer and a divisor of \(600\), we start with \(v_p = 12\) (as suggested by the solution).

8. **Substitute \(v_p = 12\):**
   Substitute \(v_p = 12\) into the equation:
   \[
   d \left( \frac{1}{v_s} + \frac{1}{v_d} \right) + \frac{600 - 2d}{12} = 50
   \]
   Simplify:
   \[
   d \left( \frac{1}{v_s} + \frac{1}{v_d} \right) + 50 - \frac{2d}{12} = 50
   \]
   \[
   d \left( \frac{1}{v_s} + \frac{1}{v_d} - \frac{1}{6} \right) = 0
   \]

9. **Solve for \(v_s\) and \(v_d\):**
   Since \(d \neq 0\), we have:
   \[
   \frac{1}{v_s} + \frac{1}{v_d} = \frac{1}{6}
   \]
   Multiply through by \(6v_s v_d\):
   \[
   6v_d + 6v_s = v_s v_d
   \]
   Rearrange:
   \[
   v_s v_d - 6v_s - 6v_d = 0
   \]
   Add \(36\) to both sides:
   \[
   v_s v_d - 6v_s - 6v_d + 36 = 36
   \]
   Factor:
   \[
   (v_s - 6)(v_d - 6) = 36
   \]

10. **Find Integer Solutions:**
    The pairs \((v_s - 6, v_d - 6)\) that multiply to \(36\) are:
    \[
    (1, 36), (2, 18), (3, 12), (4, 9), (6, 6)
    \]
    Thus, the possible pairs \((v_s, v_d)\) are:
    \[
    (7, 42), (8, 24), (9, 18), (10, 15), (12, 12)
    \]

11. **Check Validity:**
    Since \(v_s < v_p < v_d\), we discard \((12, 12)\). The valid pairs are:
    \[
    (7, 42), (8, 24), (9, 18), (10, 15)
    \]

The final answer is \( \boxed{ (v_s, v_p, v_d) = (7, 12, 42), (8, 12, 24), (9, 12, 18), (10, 12, 15) } \).