Operating system processes
50% I/O wait time means that a process is not in execution(i.e. CPU is sitting idle) for 50% of the total time a process requires from CPU to complete itself(its execution). Thus CPU Utilization turns out to be 
//whereas 50% I/O time means it needs 50% of total execution time(10 minutes) to complete its I/O.

50%=50/100 = .5

thus the time needed to complete a process which requires 10 minute of CPU will be = CPU time required by process/CPU utilization=

10/CPU utilization=  10/0.5 =  20 minutes.

when two processes run sequentially(one after the other) then the total time required will be =  10/0.5 + 10/0.5 =  20+20=40 minutes (ANSWER)

in case of parallel execution we again find the CPU utilization..
since two processes are in parallel thus the formulae becomes-->

{1-(I/O time)^no. of processes in parallel execution}
=1-(0.5)^2=1-.25=0.75
now the CPU utilization for 1 process will be 0.75/2=0.375
Therefore the time required will be =
CPU time required by process/CPU utilization=
10/0.375=26.67 minutes.

Since the two processes are running in parallel thus the time required by 1 process will be the total time required by 2 process=26.67 minutes(ANSWER)