The question asked here comes under "Interrupt-driven I/O " .
In this question " on a continuous basis" is mention , that means CPU does not have to wait for the device , when that device is going to prepare or transmit data . So that 'waiting' component is NULL . That is why, we need to consider only the time CPU is busy due to the particular I/O device
if "on a continuous basis" is not given then, (x/x+y)*100 have to use because it comes under " programmed I/O " , in this situation the CPU waits for a long time for the I/O device to be ready for data transmission.
so in this second case we should consider x/x+y where y = time taken by io device to prepare data. and x=time taken by cpu in interrupt processing
hence need to add both x and y .
But in first case while CPU is not waiting we only consider x/y .