Three years ago on my birthday, I put up a post of 5 fun math related birthday things. One of these was the “Cheryl’s Birthday” math problem which had gone viral the year prior. Here it is:
I was thinking about this recently, and found out it now had its own Wikipedia page.
The Wiki informed me that there had been a follow up problem released by the same university:
Albert and Bernard now want to know how old Cheryl is.
Cheryl: I have two younger brothers. The product of all our ages (i.e. my age and the ages of my two brothers) is 144, assuming that we use whole numbers for our ages.
Albert: We still don’t know your age. What other hints can you give us?
Cheryl: The sum of all our ages is the bus number of this bus that we are on.
Bernard: Of course we know the bus number, but we still don’t know your age.
Cheryl: Oh, I forgot to tell you that my brothers have the same age.
Albert and Bernard: Oh, now we know your age.
So what is Cheryl’s age?
It’s a fun problem if you have a few minutes. I thought it was easier than the first one, but still requires actually sitting down and doing a few steps to get to the answer. Very hard to short cut this one. It also retains the charm of the original problem of making you flip your thinking around a bit to think about what you don’t know and why you don’t know it.
The answer’s at the bottom of the Wiki page if you’re curious.