processes response time confusion An OS contains 10 identical processes that were initiated at the same time.Each process contains 15 identical requests. Each request consume 20 msec of CPU time.A request is followed by an I/O operation which consumes 10 msec.The CPU scheduling overhead is 2 msec. The system uses Round Robin scheduling with the time quantum of 10 msec.

Q1) What is the response time of 1st request of last process ? 
  A) 210 msec  B) 140 msec  c) 230 msec  D) 240 msec

Q2) The subsequent request of the processes receives a response times of 
 A) 110 msec  B) 220 msec  C) 230 msec  D) 240 msec

Ans: Q1) D

 Q2) C


What I thought : 

For 1 process, there are 15 request so 
15 * ( 20 + 10) = 450 msec  But all answers are so small than this approach. So no need to think about further i.e. CPU overhead then 2nd...3rd...processes. 

Here my problem is I didn't get the concept behind this question properly. CPU overhead ( i.e context switching ) will take place between 10 processes or each process's 15 request.
So please tell how this scenario will work. 

I didn't get the meaning of 2nd question.
Hint : Try to understand more about context switching. Context switching takes place between 10 processes. (And you can forget about those 15 requests, instead consider each process needs 15 * 20 msec of CPU time)