$12$
10. In the race between the tortoise and the hare, the hare's and tortoise's speed ratio at the start was $3: 2$. When the hare reached the pavilion along the way, it found that running another 3 kilometers would complete half the race, so it had a good sleep. When the tortoise passed the pavilion, it increased its speed to twice the original and continued on. When the hare woke up, it found that the distance the tortoise had exceeded it was equal to the distance it had exceeded the tortoise when it went to sleep. The hare regretted it and immediately increased its speed to 1.5 times the original to catch up with the tortoise, and they both arrived at the finish line at the same time. The total length of the race was $\qquad$ kilometers.