SOLUTION: Determine the number of combinations of 9 things taken 7 at a time.

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9C7= [9!]/[(9-7)! * 71]
=[9*8]/[1*2]
Notice that 9C7 is the same as 9C2
In each case you have "9" to choose from.
9C7 means you are forming groups of size 7.
But that implies you are automatically forming groups of 2.
Therefore 9C7=9C2
Each one is simply 9*8/1*2=36
Cheers,
Stan H.