The fast, medium and slow cars start at the same place at the same time and chase the cyclist in front along the same road. The three cars take 6, 10 and 12 minutes to catch up with the cyclist respectively. Given that the speeds of the fast and slow cars are 24 km/h and 19 km/h respectively, find the speed of the medium speed car.
快、中、慢三辆车同时同地出发，沿同一公路去追赶前面一骑车人，这三辆车分别用6分、10分、12分追上骑车人．已知快、慢车的速度分别为24千米/时和19千米/时，求中速车的速度．
<result>20</result>