MHB Combination Problem: C(70,67) = 54,740

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The combination C(70,67) equals 54,740 because it calculates the number of ways to choose 67 items from a set of 70. The formula used is C(n,r) = n! / (r!(n-r)!), which simplifies for this case to C(70,67) = 70! / (67! * 3!). By canceling out the factorials, the calculation reduces to (70 * 69 * 68) / 6. The final result confirms that C(70,67) indeed equals 54,740.
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How come the combination of C(70,67) is 54,740?
 
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yakin said:
How come the combination of C(70,67) is 54,740?

The combination function is defined as:

$$C(n,r)={n \choose r}\equiv\frac{n!}{r!(n-r)!}$$

And so we have:

$${70 \choose 67}=\frac{70!}{67!(70-67)!}=\frac{70\cdot69\cdot68\cdot67!}{67!\cdot3!}=\frac{70\cdot69\cdot68}{3\cdot2}=35\cdot23\cdot68=54740$$
 
There is a nice little variation of the problem. The host says, after you have chosen the door, that you can change your guess, but to sweeten the deal, he says you can choose the two other doors, if you wish. This proposition is a no brainer, however before you are quick enough to accept it, the host opens one of the two doors and it is empty. In this version you really want to change your pick, but at the same time ask yourself is the host impartial and does that change anything. The host...

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