6C4
This is a mathematical expression for combinations.
It means the combination of six items taken four at a
time, also called "six choose four" sometimes.
It represents the number of different ways to select
four items from a group of six when the sequence of
selection does not matter. The value of 6C4 is 15.
An example, using smaller numbers could be 3C2 = 3.
The three combinations of A,B,C are AB, AC, and BC.
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