CPU time is 10 minutes (given).
I/O time is 50% of the total time means total time is 20 minutes (10 minutes CPU time, 10 minutes I/O time).
Now for one process total time is 20 minutes. for two processes total time is:-
case 1: if they run Sequentially then overlapping is not allowed and total time is 40 minutes
case 2: if overlapping is allowed (Parallel processing) then I/O time of P1 can be overlapped with CPU time of P2. In this case the total time will be 30 minutes