If holes are half as large as processes, the fraction of memory wasted in holes is:
Somewhere it explained as:
Imagine processes as squares. If holes are also squares of half the side dimension of the processes (i.e. linear dimension is the largeness yardstick), then process area $= 4$ and hole area $= 1$. Then ratio of hole to total $= 1/5$ and you have your answer $(4)\space 1/5$ with a bunch of caveats.
Sorry, I didn't get the given solution, why processes as squares?
Can you explain it, please?