Given m/m cycle time=64ns
Given ,DRAM needs to be refreshed 100 times per ms
& each refresh takes 100ns
now % of mem cycle used for refreshing .
above is given data read it carefully
solution=
In every 1ms= DRAM refreshed 100 times
therefore, 10^6ns= DRAM refreshed 100 times
similiarly, 1ns= DRAM refreshed (100/10^6) times
so, (1m/m cycle) ,64ns= DRAM refreshed (100/10^6)*64 times ,which is turned out to be 64*10^(-4)
so we can conclude that in one mem cycle DRAM is refreshed 64*10^(-4) times
now, given one refresh takes 100ns
therefore, (64*10^(-4)) refreshes takes = (64*10^(-4))*100ns
which is turned out to be 64*10^(-2) ns ,that can be written as 0.64ns
basically, question is asking in one mem cycle that is 64ns,how much time is used for refreshing which is 0.64ns
in terms of percentage=(0.64/64)*100%
=1%
hope this helps,any corrections and suggestions please let me know.