Need help! How do calculate this percentage

[lgf862005]

Consider a dynamic RAM that must be given a refresh cycle 64 times per ms. Each re-fresh operation requires 100 ns; a memory cycle requires 200 ns. What percentage of the memory’s total operating time must be given to refreshes?

if u can, plz tell me the answer and formular

 

[Kral]

lgf862005,
A refresh must take place every 1/64 mS = .015625 mS = 15625 nS. Most dyamic RAMS use one complete memory cycle for each refresh, even though the actual refresh operation may take less than a cycle (in your case 100 nS). So the percent of time used by refresh = (200/15625) * 100 = 1.28%