This is actually an aptitude question and little bit of brain requirement question.
There are two methods to solve this question .
method1 is simple but very effective to apply in isro exam where time is very important constraint. I will not discuss method 1 because you can strike this method by your own. simplyby hit and trial.
method 2: CONVENTIONAL METHOD
Suppose we have x instructions or computations and each instruction or computation takes y seconds to execute.
according to question, on one cpu 20% of x = 0.2xy sec (sequential execution) + (100 - .2xy)second = 100 seconds.
and, xy = 100 seconds( when one cpu)
let q = (100 - .2xy) seconds .
now, if two cpus then, 0.2xy + 1/2q = 50 + 0.1xy = 50 + 0,1*100 = 60 seconds (as xy = 100 seconds explained above)
Similarly, if 4 cpus then, 0,2xy + 1/4q= 25 + 0.15xy = 40 seconds.