# Help me open my safe

Edit: The first version of this puzzle was apparently too hard. After a days worth of back and forth, I am consolidating all hints, answer attempts, and comments into a restated puzzle here from the point where we collectively had gotten to. We now have actually arrived at the original version of the puzzle before I posted it, which I changed because I thought it would be too easy. I guess we'll find out...
(If you want to see the previous harder version of the puzzle, look back at the edit history.)

Restated Puzzle:

My safe has an 8 digit code. Since I was worried I would forget it, I wrote four other 8 digit codes on a piece of paper and taped it to the door of the safe. The code to my safe is the next number in this sequence:

21600991
10707681
40706771
10019491


Can you help me figure out the 8 digit code to open my safe?

Hint:

People who live in the largest countries in the world may find this question easier to solve than those who don't.

• Comments are not for extended discussion; this conversation has been moved to chat. (It showed up via auto-flag, but in the future, if you'd like, just flag it yourself!) – Aza Mar 25 '16 at 8:38

Eureka!

The trick is to

read the numbers backwards. This gives you 19900612, 18670701, 17760704, 19491001. When read as YYYYMMDD dates, these correspond to the national holidays of Russia, Canada, the USA, and China, respectively. That's the four largest countries in the world, by area. Next in line is Brazil, whose declaration of independence was on 18220907.

70902281

• Well done! That's exactly right. – TTT Mar 25 '16 at 3:01
• @TTT great puzzle! – Matthew Lau Mar 25 '16 at 13:40

This is just a continuation of my previous guess, might not be the real answer because it can't explain hint 1

83034934

Since

From Hint 4, its obvious that the password is '123456'
Since he "added" the password to the paper code, the resulting numbers are

21600991
10707681
40706771
10019491
which give the answer of 83034934

Edit:

From this website, its seems 123456 is the most common password.

• I thought the password would be obvious from Hint 4, but apparently I was wrong. Sorry, that is not the correct password. – TTT Mar 24 '16 at 19:17
• I also wonder why would the above password correspond to hint 1 ... I will re-think. – Matthew Lau Mar 24 '16 at 19:18
• @Matthew Lau: The password could also result by some kind of majority vote from these four numbers. – Gamow Mar 24 '16 at 19:37
• Is it correct that Hint 2 concerns Russia and Canada? (people who live in largest countries in the world) – Gamow Mar 24 '16 at 19:39
• @Gamow - yes, among others. – TTT Mar 24 '16 at 19:42

This is my best guess: