I got the most number of correct answers for this puzzle. I was particularly impressed by Ameya Kulkarni (from London), Salil Kuchlous (from Bangalore) and Tishya Chaudhry and Anisha Sharma Goyal (both Gurgaon) – all kids who solved the puzzle.
The puzzle had appeared originally in MIT’s Technology Review, which was written by Allan J. Gottlieb, and the solution appeared in the February 1968 issue. The solution is more a trial and error process, with a few rules applied to it. I have recorded the solution as always. If anyone would like to have a more complete proof, I am happy to send it to them – please write to me directly at email@example.com.
The correct answer is 6,210,001,000. This is a unique answer for 10 digits. For fewer than 10 digits, the answers are 1210, 2020, 21200, 3211000, 42101000, 521001000.
Thanks for trying the puzzle!