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# probability sampling problem watch

1. I have a problem on the last part of a question on my intro to probability module:

I make an unordered selection of r numbers from the set {1,2,3,...,n} without replacement and with all possibilities equally likely.

What is the probability that the number 1 is in my selection, in terms of n and r. Show your working, simplify your answer as much as possible.

Thanks in advance. It's probably obvious and I'm just rusty.
2. (Original post by confused.me)
I have a problem on the last part of a question on my intro to probability module:

I make an unordered selection of r numbers from the set {1,2,3,...,n} without replacement and with all possibilities equally likely.

What is the probability that the number 1 is in my selection, in terms of n and r. Show your working, simplify your answer as much as possible.

Thanks in advance. It's probably obvious and I'm just rusty.
I'm not 100% sure but here's what I'd do:
Spoiler:
Show
P(1 in selection) = 1 - P(1 not in selection)
P(1 not in selection) = (n-1)/n x (n-2)/n x . . .

Can you work out the rest?

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Updated: April 1, 2013
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