Hey smyatsallie.

Try adding each in pairs.

First you have 20 then 2*0.4*20 (up and down 1) then 2*0.4*0.4*20 (up and down 2) and keep going to the 6th up and down which is 2*0.4^6*20.

Then you add all of these up to get your total vertical distance. See if you can calculate the distances for each of the up and down pairs and then add them all up along with the initial 20 m drop.