# Probability: Exam Forms

• Apr 10th 2012, 07:13 AM
Aryth
Probability: Exam Forms
A folder contains 10 copies each of forms A,B,C,D, and E of a college algebra final exam. If an absent minded professor pulls exams out of the folder at random, how many will he have to pull out in order to be guaranteed that he has five of the same form?

If someone could help me get started that would be great. I am absolutely horrid at these types of problems...
• Apr 10th 2012, 07:23 AM
Plato
Re: Probability: Exam Forms
Quote:

Originally Posted by Aryth
A folder contains 10 copies each of forms A,B,C,D, and E of a college algebra final exam. If an absent minded professor pulls exams out of the folder at random, how many will he have to pull out in order to be guaranteed that he has five of the same form?

HINT: $\displaystyle 4\cdot 5=20~.$
• Apr 10th 2012, 07:35 AM
Aryth
Re: Probability: Exam Forms
Oh... I get it now. Worst case is that the professor cycles through the forms in 5 draws. Which means that after 20 he would have 4 of each, making the next draw a guarantee that there will be 5 of one form.

Thanks a lot.