In 1 sec ------------------------- 100000 instructions are executed
100000/200 = 500 read and write sectors are required.
for each read or write 100 microsec required.
time for 500 read and write = 500*100*2=100000 micro sec = 0.1 sec
in 1sec only 0.1 sec is useful time
time spend in wait state = 0.9 sec
%of time spend in wait state = 0.9/1 *100 = 90%