Uncle rides a motorcycle to chase the tractor. If you chase at a speed of 30 km/h, it will take 30 minutes to catch up, and if you chase at a speed of 40 km/h, it will take 20 minutes to catch up. How long will it take to catch up at a speed of 50 km/h?
叔叔骑摩托车去追拖拉机．如果以30千米/小时的速度去追，那么需要30分钟追上，如果以40千米/小时的速度去追，则需要20分钟追上．如果以50千米/小时的速度去追，需要多长时间追上？
<result>15</result>