Device interrupts for every byte and it can send 16KB/sec at a time, so time taken to send 1 byte= 1/(16*103)= 0.0625msec
Now, it is given that interrupt processing takes time= 50 micro sec
So, fraction of time consumed by I/O device if it interrupts for every byte= time taken to process interrupt / time taken by device to send data= 50 * 10-6 / (0.0625 * 10-3) = 0.8
P.S as per hint in question (upto 3 decimal places).. so they are considering 1KB= 210 bytes. Hence, answer might vary accordingly i.e 16*210 * 50*10-6 = 0.819