Maths Olympiad Prep

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, 2014

Algebra Difficulty 4.0 AIME Find the answer United States

Problem:
How many integers nn in the set {4,9,14,19,,2014}\{4,9,14,19, \ldots, 2014\} have the property that the sum of the decimal digits of nn is even?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
Answer: 201
We know that 2014 does not qualify the property. So, we'll consider {4,9,14,,2009}\{4,9,14, \ldots, 2009\} instead. Now, we partition this set into 2 sets: {4,14,24,,2004}\{4,14,24, \ldots, 2004\} and {9,19,29,,2009}\{9,19,29, \ldots, 2009\}.

For each, the first and second set are basically x4x4 and x9x9, where x=0,1,2,,200x = 0, 1, 2, \ldots, 200, respectively. And we know that for each value of xx, xx must be either even or odd, which makes exactly one of {x4,x9}\{x4, x9\} have even sum of decimal digits. Therefore, there are in total 201 such numbers.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.