Given,data rate of 3MB per sec and 4 Byte is transferred at a time
So no of times hard disk is accessed in a second = 3MB/4B=750000
Now, total time take to service 1 transfer=1200*1ns(Clock Time)
No of times Hard disk will be accessed in 1ns=750000*10^(-9)=75*10^(-5) times
Total time taken to service hard disk transfer=1200*75*10^(-5)nsec
It's also given that hard disk is only active for 5% of time =0.05*1200*75*10^(-5) =0.45nsec
% CPU time spent on hard disk=(0.045nsec/1nsec)*100=4.5%