You’ve found Leonardo da Vinci’s secret vault, secured by a series of combination locks. Fortunately, your treasure map has three codes: 1210, 3211000, and… hmm. The last one appears to be missing. Can you figure out the last number and open the vault?

In this TED-Ed by Tanya Khovanova, autobiographical numbers, also known as self-descriptive numbers, help us solve the riddle: “An autobiographical number is a number N such that the first digit of N counts how many zeroes are in N, the second digit counts how many ones are in N and so on. In our example, 1210 has 1 zero, 2 ones, 1 two and 0 threes.”

Watch the video and then pause at these instructions to find your solution:

Find the bonus riddle solution at Brilliant, TED-Ed’s sponsor.

Next: More videos about numbers, including Universal Calendar Puzzle: Figure out the day of the week for any date ever, Can you solve the dark coin riddle? and can you solve the penniless pilgrim riddle?

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