I grew up in North America (where 911 is the emergency number), so when I moved to Germany and wanted to remember 112, I noticed that it spells out:

1 + 1 = 2

That would foil me if it was my only memory device. A+B=C is true of ~55 (napkin math) different 3-digit numbers. Plus another 40-ish for A×B=C and 50-ish for A-B=C...

But it’s A+A=B

The good news is, there’s not that many of those. 112, 224, 336, 448, 550 (its a stretch), and you’re likely to start at the beginning, therefore getting it on the first try.

Actually, I’ve heard that on cell phones in EU, you can still dial 911, and it’ll connect. I have had occasions to call 112 before, but it’s always been in big enough emergencies that I didn’t have time to experiment. I’m eagerly awaiting the day when I come across an emergency that’s urgent enough to call 112, but not urgent enough that it can’t wait a little bit for me to try something. It’s a delicate balance

In another recent HN thread, someone posted that due to the way emergency dialing is implemented in the cellular stack, dialing "911" will dial the local emergency number because "911", "112", etc. aren't treated as actual phone numbers, they just trigger the cellular network to bypass normal dialing sequence and connect you to the local emergency system.

(assuming it's A + A = B, there are still 4 sequences that could be if every number is an integer between 0 and 9, but if they remembered one of the numbers was a 1 it would drop the possibilities down to only 1 + 1 = 2)