Lets Assume for 1 second of cpu time,
cpu time = main program execution + time to service some no of Interrupts
we know in 1 sec there are 10^3 msec, so there are 10^3 interrupts arriving in 1 sec and from diagram we know
1 interrupt takes = 10 usec(saving of cpu state)+80 usec(ISR execution)+10 usec(Restoration of cpu state)
so total 1 interrupt takes 100 usec which is 10^-4 sec
so time to service 10^3 interrupt takes 10^3 * 10^-4 = 10^-1 sec
so Main program execution=cpu time - time to service 10^3 interrupts
Main program exec=1-0.1 = 0.9 sec
which is 90% of cpu time for this assumption