D.3 Walk around the pond

Taro of 50m walk per minute and Jiro of 30m walk per minute started at the same time in the same direction around a circular pond. 

When Taro who separated Jiro catches up with Jiro in five minutes, find the distance around this pond?

Answer

100 m

Solution

Assume that Taro of one lap of delay catches up with Jiro. 

Because the distance to catch up with in one minute is 50m - 30m = 20m and Taro caught up with Jiro in five minutes, far distance is 20m × 5 minutes = 100m which is the distance around the pond.