Question:

Suppose you are applying to graduate schools. Your chances to be admitted to each school are 30% and are the same for any school. To how many different schools you need to apply if you want chances to be admitted to at least one school to be above 90%?

I think that it relates with confidence intervals, but I'm not sure how it is possible to calculate n, the number of graduate schools.

I'd appreciate your help if you'd provide me with a formula that is appropriate for this kind of question.