First line:I m assuming there is only 1 question ...so 2^1-1 students will get 1 question wrong ..which gives 1 question wrong

Secong line:Let there be 2 questions ...2^2-1 students get 1 questin wrong ., 2^2-2 students get 2 questions wrong ..thus 3 questions totally wrong ..

This way if we see ..in general ..for n questions ..there are 2ⁿ -1 wrong answers ..