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)