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Problem of the Day #300: Lucky Days (a.k.a. Friday the 13th) January 13, 2012

Posted by Alex in : potd , trackback

Suppose the calendar days were numbered starting from $1$, and day $1$ is a Friday. A day is lucky if it is a Friday and the sum of the digits of its number is a multiple of $13$. How many days from $1$ to $1,000,000$ are lucky?

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