61.If a cyclist rides at a speed of 10 km/h, he will be 1 hour late. If he rides at a speed of 15 km/h, he will arrive 1 hour early. At what speed should he ride to arrive on time?
61.If there were two cyclists, with the first one's speed being $10 \mathrm{km} / \mathrm{h}$ and the second one's speed being $15 \mathrm{km} / \mathrm{h}$. Then, according to the problem, if the first cyclist started 2 hours earlier than the second, they would arrive at the destination simultaneously. In this case, in 2 hours, the first cyclist would cover 20 km, and the second cyclist would be able to catch up in 4 hours - already at the final destination. Therefore, the entire distance is 60 km. The question of the problem can be rephrased as: "At what speed should a cyclist travel to cover the entire distance in 5 hours?". Answer: 12 km/h.