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View Full Version : HELP studying for a FINAL EXAM

Ayisha
03-20-2006, 05:39 PM
Hello everyone, I have a final that I am studying for, the final isnt till thursday. But I am having problems soving some probability problems, I will show what I have done so far...

1) a student is to answer 15 out of 20 questions on an examination. how many choices does he have? How many if he must answer atleast 8 of the first 10 questions.

I know how to do the first part it is just nCr, where n=20 and r=15, and the answer is 15504. But the second part I an not sure on how to do, I have an idea to use nCr but I know there is something I have to do with the remaining 7 questions from the total of 20. The professor gave us the answer which is 7752 but i dont know how to get that answer.

2) In how many ways can a boy divide 10 gifts among four of his friends if his best friend is to receive 4 gifts and the others are to receive 2 each?
The answer is 18,900....But I am not sure how to go about doing this problem .

3) How many different 7-character passwords are possible when 3 of the characters are letters and 4 are digits? Assume that the repetition of letters and numbers is allowed and that there is no restriction on where the letters or numbers can be placed.