2006 AMC 10A Problems/Problem 20

Problem

Six distinct positive integers are randomly chosen between 1 and 2006, inclusive. What is the probability that some pair of these integers has a difference that is a multiple of 5?

$\mathrm{(A) \ } \frac{1}{2}\qquad\mathrm{(B) \ } \frac{3}{5}\qquad\mathrm{(C) \ } \frac{2}{3}\qquad\mathrm{(D) \ } \frac{4}{5}\qquad\mathrm{(E) \ } 1\qquad$

Solution

For two numbers to have a difference that is a multiple of 5, the numbers must be congruent $\bmod{5}$ .

$0 , 1, 2, 3, 4$ are the possible values of numbers in $\bmod{5}$ .

Since there are only 5 possible values in $\bmod{5}$ and we are picking $6$ numbers, by the Pigeonhole Principle, two of the numbers must be congruent $\bmod{5}$ .

Therefore the probability that some pair of the 6 integers has a difference that is a multiple of 5 is $1 \Rightarrow E$ .

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2006 AMC 10A Problems/Problem 20

Problem

Solution

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