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