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# The Number of the Day: 40,000

Of the 600,000 troops who invaded Russia under Napoleon in 1812, fewer than 40,000 made it back to France.

Related Link: This week we're pulling Numbers of the Day from the beautiful and fascinating National Geographic Answer Book, which hit stores last Tuesday. You can order your copy from the National Geographic Store.

[See previous Numbers of the Day here.]

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This Puzzling Math Brain Teaser Has a Simple Solution
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Fans of number-based brainteasers might find themselves pleasantly stumped by the following question, posed by TED-Ed’s Alex Gendler: Which sequence of integers comes next?

1, 11, 21, 1211, 111221, ?

Mathematicians may recognize this pattern as a specific type of number sequence—called a “look-and-say sequence"—that yields a distinct pattern. As for those who aren't number enthusiasts, they should try reading the numbers they see aloud (so that 1 becomes "one one," 11 is "two ones," 21 is "one two, one one,” and so on) to figure the answer.

Still can’t crack the code? Learn the surprisingly simple secret to solving the sequence by watching the video below.