Math Is Fun Forum

chetah

Member

Registered: 2008-02-15

Posts: 32

Discrete Math

Calculate 6
              2
               
Check your answer by listing all the selections of size 2 that can

be made from the letters a, b, c, d, e, and f.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Discrete Math

I'm not sure whether you mean 6C2 or 6P2, so I'll tell you how to do both.
The formula for nCr is n!/[r!(n-r)!]. For n=6 and r=2, that means 6C2 = 6!/[2!*4!] = 15.

The selections of letter from a to f would be ab, ac, ad, ae, af, bc, bd, be, bf, cd, ce, cf, de, df, ef - 15 in all.

If you want nPr, then the formula is just n!/(n-r)!.
There will be more selections here, because this time ab and ba count as two different selections.

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