Let time taken for a single task=1 sec
Then ,Time taken to perform 8 tasks on a single processor =8 as they have to be executed one after another.
Now, when there are 4 processor,(Say P1,P2,P3 and P4)
then allocation of tasks is done in this manner
At t=2, P1-T2
At t=3 ,P1-T3 P2-T4,P3-T5
At t=4,P1-T6 P2-T7,P3-T8
So total time taken=4s