Calculating the probability of getting less than the expected value

If your taking a multiple choice test (4 options) and there are a hundred questions, you would expect to get about 25 questions right by random chance. But you could get unlucky. you might get only 20 right by random chance. How can a calculate the chance of getting even less than the expected value? I don't seem to be able to recall the formula or the name of this type of probability calculation. I presume it has something to do with a Z-score, but idk.