One day a robber got into a bank to rob its vault. However, he noticed that the vault had an unusual lock. So he hid behind a desk and watched how others got in. He noticed that the manager came up to the vault door and pressed a button. The lock read out a number in response to which the manager said a number of his own. On doing so the door opened. This happened twice. The first time the lock said 12 and the manager responded with 6. The second time the lock said 6 and the manager responded with 3. Believing that now he knew all that he needed to know, the robber approached the door. The door said 10 and in reply the robber entered 5. It set of the alarm and the robber was apprehended.
What number do you think the robber should have said?
Answer on Monday.