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This is a simple probability question.Let us think like this :

Let us think of complementary event i.e if no process is in CPU at an instant i.e. CPU being idle . For that it is necessary that all 3 processes should execute their I/O time and not CPU time.

Given I/O time for each process = 25 % = 0.25

So to execute I/O time simultaneously by 3 processes , by multiplication principle,

probability(all perform I/O operation) = 0.25 * 0.25 * 0.25 = 0.015625

So probability of CPU time being utilised = P(at least one process in running (CPU active) state)

                                                           = 1 - P(all perform I/O operation)

                                                           = 1 - 0.015625

                                                           = 0.984375

                                                           = 0.98(approx)
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