Permutation and Combination - Number of selection of r things

Dear experts,

Can you please refer me to any online material/book which will demonstrate the application of the below rule with examples.
Please help.

Rule:Number of selection of r things (r<=n) out of n identical things is 1

Thank you
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ste5anSenior DeveloperCommented:
Huh? When you have a bag of n identical marbles, how many different ways do you have to draw two marbels?

Combination: Order or sequence does not matter.
Permutation: Order or sequence does matter.

For example:
Bag (n=3) of (Red, Red, Red)
The only possible result for drawing two marbles is: Red, Red. Cause there is no other combination or permutation.
All combination and permutation problems are normally reduced to sets. Thus each element is different from each other.

Bag (n=3) of (Red, Blue, Green)
Possible draws of two: RB, BG, RG ( and BR, GB, GR) => 3 combinations or 6 permutations

The only problems arise in mix forms

Bag (n=3) of (Red, Blue, Blue)
Possible draws of two: RB, RB, BB ( and BR, BR)  => 2 combinations or 3 permutations.

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"Rule: Number of selection of r things (r<=n) out of n identical things is 1"
the Number of selection of r things (r<=n) out of n identical things is 1 only if r = n.
The fact the all n are identical can cause trouble with interpretation.
ste5an addresses this problem correctly
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