You Didn't Tell Me Any Of These Numbers!

Number Theory Level pending

I have 6 consecutive positive integers. If the product of these 6 numbers is divisible by precisely 3 of the numbers below, then this product is not divisible by ______ \text{\_\_\_\_\_\_} .

7 , 8 , 9 , 10 7, 8, 9, 10

9 7 8 10

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2 solutions

Zee Ell
Sep 29, 2016

A set of 6 consecutive integers always contains:

• exactly 3 even numbers, hence their product has to be divisible by 8 (and also by 16, as at least one of these even numbers is divisible by 4 as well)

• exactly 2 numbers, which are divisible by 3, therefore their product has to be divisible by 8

• at least one (and at most 2) number, which is divisible by 5 and since we have 3 even numbers in the set as well, their product has to be divisible by 10

• we do not necessarily have a number in the set, which is divisible by 7, e. g. the set {1, 2, 3, 4, 5, 6} does not contain any and therefore neither is their product divisible by 7 (in general, we can take the set {7k + 1, 7k +2, 7k + 3, 7k + 4, 7k + 5, 7k + 6} as counterexample)

Hence, our answer is: 7 \text {Hence, our answer is: } \boxed {7}

Kushal Bose
Sep 30, 2016

Product of n consecutive numbers is always divisible by n!.So,here 6 consecutive numbers is divisible by 6!=720 which has factors like 8,9,10 but 7 is not there. So our answer is 7

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