Number Trick - Numberphile

Just a quick number trick with Dr James Grime.
More links & stuff in full description below ↓↓↓

NUMBERPHILE

Videos by Brady Haran

[cizma]

I have a tip for you and for all those who are performing this. If your spectator's number has two or more same digits like (7721) it is kind of suspucious for you to write 2278. But in order to make it u suspicious you could for example substract 1000 from your number and write 1278. And then in the next step you add that 1000 to your number. You dont even have to add/sub that simple number. You could for example substract 1324 but then you just must add it to another number. And that's it. Simple tip to make this trick even better. I hope you like it

[dimmitsaras]

I noticed there was a scum going on with your numbers when i saw 22 responding to 77, which made me realise that your numbers werent random after that point.

[richard_pine]

I went to try this, and the person said 8, 888. And I said 1, 111. Then they knew these weren't random, and she wasn't impressed.

[alakadaspatnaik]

I knew this trick but never understood it, this guy made me understand it and have fun

[numberphile]

I feel like my Mum is telling me she is not angry, just disappointed!

[DlcEnergy]

That was such an awesome, yet simple trick. If you don't explain it to someone, they'll be impressed because they won't know how you did it.

[EnglishRain]

I'm still at 0:08 and I paused to thumb you up for that amazing English accent!

[SupaDanteX]

Some other interesting things may include:

If you square a number of ones, you get to count up and down based on how many you used. For example.

1x1 = 1
11x11 = 121
111x111 = 12321
1111x1111 = 1234321
...
...


And if you square a number ending in 5, you take the extra bit (n), multiply it by n+1, and then put 25 on the end (because the 5 in the ones place is 5x5=25)

So for example.

35 x 35.

n(3) +1 = 4.

n(3) x n+1(4) = 12.

So 35 x 35 = 12 (and then put the 25 on the end) 25.
35 x 35 = 1225.

Or another example,

75 x 75.

7 + 1 = 8.
7 x 8 = 56

75 x 75 = 5625.

[gdibble]

3:08 "and you can look like a mathematical genius" XD

[DRoo]

The outcome DE...F can be written als 10000*D+1000*E+...+F = (9999+1)D+(999+1)E+...+F. As 9999, 999 etc. are all divisible by 9, the leftovers from dividing by 9 must be equal to D+E+...+F. So if DE...F is divisible by 9, so is D+E+...+F. (I thought this could be proven nicely using modular arithmetic but I don't have the knowledge for that.) So once knowing the mixed up version of the outcome, you just add each digit and figure out which digit you need to make the total divisible by 9. (2/2)

[YogiliciousP]

Wow...I watched this a while back and it went right over my head. Now the while back was closer to the time when I suffered a concussion, actually a double concussion. Today I get it...totally simple too! I was having quite a difficult time with simple math...for example, military time is easy to do, sans a tbi. The midnight to noon are as is. The 1300 to 2359 are simple if you take the first two numbers, subtract 2, drop the first number if it's a 1 OR subtract 1 if it's a 2, and that's your hour, now just carry over the minutes. Alternatively, subtract 12, or 2 then 10. I not only couldn't do it, but I couldn't explain it.
Ex: 1838 --> 18 - 2 = 16, drop the 1 in the tens place --> 6pm carry over the minutes --> 6:38pm.
Ex: 2342 --> 23 - 2 = 21, now subtract 10 = 11, carry over minutes as is 11:42pm. 
I had expected to join the army and have spent the last several years with calendars and clocks in military time, expecting military time to become second nature...I still have to do the math jog. The math jog was what became second nature until I bumped my noggin!

[awesomex-uzim]

Omg my old math teacher used to do this. He even once wagered his iphone and said if he is wrong he'll give it to the student😂

[numberphile]

yes, though you need that control number at some point... I think the way James wrote it as an "example off the top of his head" conceals its purpose quite well and made it feel natural!?

But the fun of these tricks is adapting them and making them better!

[greggwithg]

This is fun - I like starting with a 4-digit number ending in 1 so that the resulting 5-digit sum has even fewer digits in common with the starting one, making the pattern harder to spot!

[numberphile]

If someone picked 9999 I'd do something like "oh being cheeky are you going for the biggest - just for that I'm picking 0000, ha ha"and make a little joke of it, etc!

[DRoo]

Here's how it works: If you subtract the digits from a random number (AB...Z) the following happens: = = + 1111*B + ...). As you can see, the result will always divisible by 9, and so will the outcome of the multiplication be. (1/2)

[TheWolf]

oh, the math professor doesn't use an iPhone
now i truly believe that he's and all-around smart man

[facebren]

Or write 1000. For the next number, you just make the first digits add up to 8

[numberphile]

no... I am actually really enjoying myself today! must get back to some editing though...

[noodlesthest]

too easy to figure out on the spot I think.