2. From one point on a straight highway, three cyclists start simultaneously (but possibly in different directions). Each of them rides at a constant speed without changing direction. An hour after the start, the distance between the first and second cyclist was 20 km, and the distance between the first and third - 5 km. At what speed is the third cyclist riding, if it is known that he is riding slower than the first, and the speed of the second is 10 km/h? List all possible options.
Answer: 25 km/h or 5 km/h.

Solution. Let's draw a numerical axis along the highway, taking the starting point of the cyclists as the origin and directing it in the direction of the second cyclist's movement. His speed is 10 km/h, so after an hour, he was at point B with a coordinate of 10. The distance from him to the first cyclist is 20 km, which means the first cyclist was at point $\mathrm{A}_{1}$ with a coordinate of 30 (if he was moving in the same direction as the second) or at point $A_{2}$ with a coordinate of -10 (if he was moving in the opposite direction). Let's determine the coordinate of the third cyclist. The distance between him and the first cyclist was 5 km. However, since his speed is lower, he could not have moved further from the starting point. Therefore, in the first case, he was at point $\mathrm{C}_{1}$ with a coordinate of $30-5=25$, and in the second case, at point $C_{2}$ with a coordinate of $-10+5=-5$. Accordingly, in the first case, his speed is 25 km/h (and all cyclists are moving in the same direction), and in the second case, his speed is 5 km/h (and the direction of movement of the 1st and 3rd cyclists is opposite to the direction of the 2nd cyclist).

Criteria. Only the answer (if both options are specified) - 1 point. If only one case is considered in the solution - no more than 3 points.