ACT Math · Probability and counting

How many different ways can 6 numbers be chosen from a set of 49, without regard to order?

How many different ways can 6 numbers be chosen from a set of 49, without regard to order?

Answer and explanation

Answer: B: 13,983,816

Choosing 66 numbers without regard to order is a combination: (496)=49!6! 43!=49⋅48⋅47⋅46⋅45⋅44720=13,983,816\binom{49}{6}=\frac{49!}{6!\,43!}=\frac{49\cdot48\cdot47\cdot46\cdot45\cdot44}{720}=13{,}983{,}816, choice B. Choice C (49649^6) counts ordered choices with repetition allowed, so it is far too large. Choice D (6496^{49}) swaps the base and the exponent. Choice A (294294) is 49⋅649\cdot6, a product that does not model any selection. Because the order of the numbers does not matter, we divide the permutations by 6!6!, the number of ways to order six numbers.

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