Memory cycle time: Time between any two consecutive memory operations. Here it is given as 64 nanosecond.
The questions says “Dynamic ram” which implys that the the memory unit needs to be refreshed constantly to maintain its state, otherwise it will loose its contents. But how fast we need to refresh it, it is also given as “100 times per millisecond”.
converting everything to nanosecond:
→ In 1 millisec we do 100 refreshes
→ In $10^{6}$ nanosec we do 100 refreshes. (1 millisec = $10^{6}$nanosec)
→ In 1 nanosec we do $\frac{100}{10^{6}}$ refreshes (0.0001 refreshes)
So in 64 nanosec we do 0.0001 * 64 = 0.0064 refreshes.
This means that out of our 64 nanosec memory cycle time 0.0064 nanosec is spend only on refresh operation, the ques asks this value only, but in percentages.
$\frac{0.0064}{64}*100=1$%