To answer the first 15 questions with true or false there are 2^15 ways

And for the remaining 5 MCQs with 4 choices each there are 4^5 = 2^10 ways to answer them.

So there are 2^15 × 2^10 = 2^25 (33,554,432) ways to answer the exam. This means

Acing the exam by sheer chance is 1/2^25 = 1/33,554,432 =2.98023224 × 10^(-8) ie., very slim.