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Similar questions give 27% - 2 students and 89% - 10 students

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Answer was 17?

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94% chance the first student opened one.
94%*94% chance both the first two students opened one.
94%^3 chance of the first three, etc...

When it gets to student #12:
0.94^12 = 0.476

It takes 12 students before there is greater than a 50% chance that one of them has opened an offer.

The general formula is:
1 - 0.94^x
where x is the number of students.  0.94^x is the chance that x number of students have all opened an offer.  So 1-0.94^x is all other chances, that is, that at least one student has opened an offer.