The key to this problem is to convert all units uniformly when calculating! Otherwise, you can't get the correct result. 20÷ 1000 is to convert 20 meters into kilometers, and 1.5÷3600 is to convert 1.5 seconds into hours.
Difficulties: How to determine the point M? Because the side lengths of the left right-angled triangle are 3, 4 and 5 resp