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My worst math questions are the probability ones. I always skip them w/ out even looking at them. These and sometimes some hard geometry ques are only ones that I actually skip.

Help me out on this one:

Lisa shakes her piggy bank which contains 5 quarters and 5 nickels until 5 coins fall out with equal probability, what is the probability that the sum of the change that has fallen out is $0.85?

So far, all I know is that 3 Q and 2 N can only add up to $.85 and I'm just lost after that.

This problem can be solved with combinations. Like you said only 3Q and 2N can work.
You have 5Q but you only want 3Q. So you can make 5nCr3 combinations.
You have 5N but you only want 2N. So you can make 5nCr2 combinations.
When you multiply these you get the number of combinations that results in a sum of .85.

But since you want probably you have to get the total number of combinations. 10 coins total but you only want 5. So there are 10nCr5.

So your answer should be 5nCr3*5nCr2/10nCr5=25/63

Man - you just defied the law of MAth Marathon! Please. Let us ALL contribute to this amazing feature of this forum... Post ALL your questions over there - at Math Marathon

Here is the link if you don't know...

viewtopic.php?f=4&t=3450

Also, follow the instructions on that page!

Box A contains 6 red balls and 2 green balls. Box B contains 4 red balls and 3 green balls. A fair die is thrown. If an even number is obtained, a ball is selected from box A; if an odd number is obtained, a ball is selected from box B. If a red ball is selected, what is the probability that it came from box B?

Anyway, I'm just trying to expand my knowledge on prob by doing different types of them.

Help on Inv. A 2012 #22