Any mathematicians out there know how this math trick might work?

October 26, 2018 9:27 AM

It's a math trick that involves using someone's time of birth to find out their birthday

It was a really cool trick that I saw on a trip once. I wish I could remember all of it. You ask a person for their birth time and if they don't already know it you tell them they can find it on their birth certificate. ex 8:42pm- (maybe army time was used, I don't remember for sure) With these numbers you then do a series of mathematical thingies that ultimately tells you when the person's birthday is. And then the person gets their mind blown. Can someone think of how a trick like this would work considering I don't remember the details of the mathematical thingies that come after?

posted by fantasticness to Science & Nature (14 answers total)

There is no possible way you could calculate the person's date of birth from just their time of birth. The two are not correlated and, for example, if someone is born on 1st Jan at 9am, that doesn't mean no-one is born at 9am on the 2nd Jan, or 3rd, and so on. There must be a bit more to it than this (i.e. there's more info given than just the time), or some trickery where the asker already knew the answer and it was a Derren Brown-style fake-out.
posted by EndsOfInvention at 9:31 AM on October 26, 2018

You ask a person for their birth time and if they don't already know it you tell them they can find it on their birth certificate.

How did this step play out? Did the participants literally look up their birth certificates in a way that allowed for a quick glance at the birthday? This is a posssible locus of the sleight of hand that must be going on (as per EndsOfInvention above). Or maybe the asker's process of "running tricky math equations" was more of a "cross-referencing person's name to a list of participants' birthdays" or some such thing!
posted by elephantsvanish at 9:37 AM on October 26, 2018

This does not seem possible, but just in case, I was born at 4:44 pm. (Hint: Look in profile.)
posted by AugustWest at 9:49 AM on October 26, 2018

I only remember that after the birth time was received the math guy did some math stuff and said random numbers like "times that number by 7" etc etc... and after a while of that came up with the birthday. Maybe it was slight of hand I dunno, but I know there are math-magic people out there who use real math to come up with these kinds of tricks.
posted by fantasticness at 9:52 AM on October 26, 2018

The only way I can see it working (other than sleight of hand) is if part of the tricky calculations includes introducing the month/day of birth into the calculation and then reversing the rest of the calculations to end up with just that information.
posted by metahawk at 9:53 AM on October 26, 2018

Here's a similar trick, but it requires you knowing your birthday...maybe that will help jog someone's memory?
posted by griseus at 9:54 AM on October 26, 2018

Typically the way these things work is that the mathematical operations hide the fact that you're really telling them your birthday, just in some obfuscated fashion. Steps like this will probably include your age at some point, or a number from which your age could be calculated. The fact that you don't remember the steps, and that they're very complicated, suggests that information was obtained in a way you didn't notice.
posted by ubiquity at 9:56 AM on October 26, 2018

at some point there's going to be a step like "take the last two digits of the year you were born, don't tell me, now add five and ..." or something along those lines.
posted by zippy at 9:59 AM on October 26, 2018

Recovering magician / mentalist here. I am 100% certain the time of birth is misdirection and window dressing for a very simple trick.
posted by The Deej at 10:00 AM on October 26, 2018

I vaguely recall something like this in school from about 30-35 years ago and I think Zippy has it.
posted by meinvt at 10:50 AM on October 26, 2018

Yep, I'd bet that it boils down to a trick like this one where the directions include adding/multiplying by the month/day of birth. If you write it out as an equation and simplify all the way, you can see what's going on. This trick just starts with the number 7, but there are other tricks that will take a random number and reduce it to a known number. If you chain these tricks (or ones that work on the same principles), you could design a trick that starts with the time of birth, introduces the month/day of birth into the equation, discards the time of birth, and ends with the month/day of birth. Make the last few steps simple enough to do in your head, and you can stop at what looks like a random number, but still know their birthday.
posted by yuwtze at 11:56 AM on October 26, 2018

The easy way to persuade yourself of this is to contemplate the fact that people are born at that time every day. With no further data, how could it be possible to distinguish amongst them?
posted by praemunire at 12:28 PM on October 26, 2018

There is a simple math trick where, if someone tells you their birthday, you can identify what day of the week it was.
posted by thelastpolarbear at 2:24 PM on October 26, 2018

I would guess that they know the day-of-week of your last birthday and your age. Then they ask your birth time (and probably day-of-week) to give them the time to do the (any day-of-week from any date math magic) to figure out the possibilities. (What recent Wed is 38 years from a Tue 38 years ago. Not sure if that is unique or not... :P) It's probably in one of those Mental Math Tricks to Impress your Friends books somewhere.
posted by zengargoyle at 12:52 PM on October 27, 2018