I have come across this question, the answer is simply stated as 36, and while I can see how 36 is gotten, I don't understand why? How is it meant to be known that b = 36, just looking at the formula is nice and quick but I much prefer to understand why the statement in the question is true?
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Given that $\binom{40}4=\frac{40!}{4!b!}$, write down the value of $b$.