Consider a skip list in which we build new towers with probability 3/4. When adding an element to the skip list, we flip two coins at the same time, and repeat this until both coins come up tails. The number of times we toss both coins defines the height of the tower (i.e., if both coins come up tails in the first flips, there will be one node in the skip list, if both come coin on the second try, the number of nodes will be 2, etc.).

Using the probability for tower heights described in the above quote, derive the expected height of a tower.