It falls down 20 meters, then up 10 meters. So its first "down+up" is 30 meters. Then it falls down 10 meters then up 5 meters. So its second "down+up" is 15 meters. So we have an infinite geometric series with a1 = first term = 30 and r = common ratio = 1/2. The formula for the sum of any infinite geometric series is:Substituting: Edwin