Posted by **katie** on Tuesday, September 12, 2006 at 4:43pm.

how do you calculate combinations [C(6,2)] without using a calculator. I know that this is supposed to be a case of choosing 2 of six objects, but I'm notfamiliar with the logic behind it. can you help, please?

It's actually 6 choose 2 objects

The definition of nCr or n choose r is

(1) n!/(r!(n-r)!)

Here n=6 and r=2 so we have 6!/(2!*(6-2)!)= (6*5*4!)/(2!*4!)= 6*5/2! = 3*5=15

The formula (1) is what you should remember here.

25

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