In the Math Corner of my April Diary I offered this.
Oh, it's a brainteaser you're wanting? I'm not sure this one really counts, but it defeated me (and has, I should say, nothing whatsoever to do with digitally delicate primes). It may in fact be one of those puzzles that divides the human race into two non-overlapping subsets. Persons of a visual-artistic inclination perhaps get it right away, while those whose thinking is more abstract and numerical overthink it, as I did.
(For all 35 years we've been married Mrs Derbyshire and I have been setting aside an evening now and then to sit and watch a rented movie, and for just exactly as long she's been grumbling that I fall asleep halfway through the show.)
Anyway, this was going round on WeChat, my lady's Chinese social network. She challenged me with it, and I failed.
Here is a false statement:
5+5+5 = 550.
Can you make the statement true by adding one single line?
Hoo boy, did I overthink it! A mighty host of readers told me that adding a line to the equals sign will do the trick:
5+5+5 ≠ 550.
That works of course. It seems not to be what the WeChatter had in mind, though. His answer was:
545+5 = 550,
turning the first plus sign into a 4 by adding a diagonal stroke in the northwest. Hardly any of my readers got that … which makes me feel a little better.
I think what actually failed here was my Chinese. Mrs D showed me that on her smartphone. I looked & mentally translated it as above. I think the question as actually posed was probably: "Can you make this a true equality by adding a single line?"