There are 20 questions in the multichoice-exam and there are 4 answer alternatives on every question.
Only one alternative is always correct.
How many different answer papers is possible to form in which there are 70% of the answers correct?
Here is my solution ( I am not sure is this same formula needed here, but it might be right ) and that 70% would be * 0,7
(20 + 4 - 1)!
__________ * 0,7 =
20! * ( 4 - 1)!