In class of 25 students 3 students failed. what is the probability that student from this class will pass (not fail)

The probability that a student will fail is 3/25, there are 25 of them, thus the sample space is 25 and 3 out of that is 3/25 or 0.12.

what's the probability that one will pass? well, the opposite of failing, thus is the complement of 0.12, namely, 1 - 0.12.

3/25 fail, so that means that 12% fail or 0.12. so that means that 22/25 don't fail so the probability of passing would be 22/25 or 0.88 or 88%.