​The puzzle...

Your new boss Adam (yes, that’s me!) starts at work.

You and your coworker Chandika ask me, “Hey Adam, when is your birthday?”

I hate a straight answer. But I love brainbusting puzzles.

So I say, “How about this? I’ll give you 10 choices for my birthday, and I’ll whisper you each a hint. I'll tell you only the month of my birthday and Chandika only the number of the day (e.g. 1st, 2nd, 3rd, etc). There's a fat bonus for the first person to tell me my birthday.”

“Okay,” you and Chandika say, “let’s play!”

“Well, I begin, “my birthday is one of the following 10 dates:

Mar 4, Mar 5, Mar 8, Jun 4, Jun 7, Sep 1, Sep 5, Dec 1, Dec 2 or maybe it is Dec 8.”

I then whisper in your ear the month of my birthday, and whisper to Chandika only the number of the day.

Once I’ve shared your hint, you think for a second and blurt out: “I don’t know Adam’s birthday, and I’m certain Chandika doesn’t know it either.”

After hearing what you said, Chandika replies: “Well I didn’t know Adam’s birthday after his hint. But now I know it.”

​

Once you hear Chandika say this you then say, “Ah - now I know it too."

While your brain is bursting trying to figure out what my birthday is, Bogdan who was walking by, saw the list, heard the rules but not the hints and heard your comments, writes the answer on a piece of paper. Damn you Bogdan!

The solution....

If the birthday was June 7 or Dec 2, Chandika would know immediately from just the number.

So when you say “I don’t know” it could be any of the 4 months, but when you then say “and I know Chandika doesn’t know either” you are saying “I have not been told June or December”.

The only possible dates are now March 4,5 or 8 or September 1 or 5.

So when Chandika says “I didn’t know” she is saying “the number I have been told is a repeated number”. But when she adds “but now I do know” she is saying “now that I know it is either March or September I do not have a repeated number”

So that removes March 5 and Sept 5

The only possible dates are now March 4, March 8 or September 1.

If you had been told March, you still would not know which of the two March dates ate the birthday. But you say “now I know”. You must have been told September.

While your head is still spinning Bloody Bogdan jumps in with “It’s September 1”

Show pony!

Yours in nerdiness,

Adam