Cyrus walked through the huge door, which leaded to a hollow tunnel. It was very dark inside, so Cyrus lit up his torch. He walked through the tunnel, but he couldn't see any exit, except for where he came from. Cyrus knew that he mustn't give up, so he searched more carefully. Suddenly, he saw two letters on the wall: A and U. Under the two letters was also a mini keyboard which only has numbers and decimal point. There were 5 blanks under the keyboard. Cyrus tried typing random numbers, but a buzzer rang when he entered 5 numbers. That must be a wrong answer, Cyrus thought. Can you solve the problem for Cyrus?


Mona Lisa

I will publish the correct answer if no one has the right answer

  • $\begingroup$ Is it necessary to have a priori knowledge from Cyrus' first adventure? $\endgroup$
    – Annosz
    Feb 13, 2019 at 16:22
  • $\begingroup$ is there a keyboard and 5 blanks under each letter, or one for both of them ? $\endgroup$ Feb 13, 2019 at 16:26
  • $\begingroup$ @Annosz No, it is not linked $\endgroup$ Feb 14, 2019 at 6:36
  • $\begingroup$ @RémiHenry There is a keyboard and 5 blanks, seperated $\endgroup$ Feb 14, 2019 at 6:36
  • $\begingroup$ @OmegaKrypton Someone has answered already... Hehe $\endgroup$ Feb 17, 2019 at 13:22

3 Answers 3


my guess is



Most people got the AU to Gold (Science) reference.

Linking Mona Lisa to Gold yields .... the Golden Ratio, which is 1.618 ....

The math behind this have many other applications other than art .... ] Source: https://en.wikipedia.org/wiki/Golden_ratio

  • $\begingroup$ Yes you are correct! $\endgroup$ Feb 17, 2019 at 13:21

By seeing your first riddle:

'A' is the 1st letter of the alphabet, the first Fibonacci number is 0
'U' is the 21th letter, and the 21th Fibonacci number is 10946


A+U -> 0+10946 = 10946

I am pretty sure, thats wrong, because of

the importance of the decimal point

but whatever... :)

  • $\begingroup$ also the mona lisa follows the golden (Au) circle - which is based on the Fibonacci sequence $\endgroup$ Feb 14, 2019 at 9:39
  • $\begingroup$ (also, in the previous challenge, 0 wasn't counted as the first number - so could be 10947) ;) $\endgroup$ Feb 14, 2019 at 9:40

Could the AU mean



You mentioned decimals which makes me think of the atomic mass of gold which includes a decimal. So the code might be 196.9 which is the atomic mass of gold.


This satisfies the use of the mathematics and science tag.

  • 1
    $\begingroup$ I was also thinking along those lines but didn't see how math fits in.. I think you are right though $\endgroup$
    – yaara
    Feb 13, 2019 at 16:53
  • $\begingroup$ This is correct, but it's not my initial answer. Think about Mona Lisa. $\endgroup$ Feb 16, 2019 at 14:47

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