Answer: Choice A
15C12
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Reason:
Imagine there are 15 students and we wish to form a committee of three people, where order doesn't matter. The answer to this problem is 15C3 using the nCr formula. Here n = 15 and r = 3.
The 15C3 represents the number of ways to pick the three people on the committee, while 15C12 is the number of ways to not pick a person for the committee. We can think of it like two separate committees if you want.
So 15C3 = 15C12
The numbers after the "C" add to the value of n
The identity is
The r and n-r add to n.