# How-To Geek

## An Awesome Number Trick to Impress Your Friends With

Want to look like a mathematical genius in front of all your friends? Then you will love this awesome number trick from Numberphile! The best part will be the looks on their faces when you surprise them by having predicted the answer ahead of time…

Number Trick – Numberphile [YouTube]

- Published 06/6/13

I love Numberphile!

A great thing with this trick is that it scales up very easily. If you want to sum a total of seven numbers, (your starter, plus three from your friend and three from you) just add 30,000 to your starter and then subtract three. For nine numbers, add 40,000 and subtract four.

You can also scale up the length of the numbers. For a total of five five-digit numbers, add 200,000 and subtract two. For five six-digit numbers, add 2,000,000 and subtract two.

Then, of course, you could combine the scaling mechanisms. So, to sum a total of eleven seven-digit numbers you would add 50,000,000 and subtract five.

To add another spin to it, you can mix up the order in a few ways. For a five-number sequence, any of the following orders could be used, where Y is You and F is your Friend:

YFYFY (as seen in the video) YFFYY FYFYY FFYYYYou just need to make sure that your complementary numbers (in this case, I use the term to mean a number that adds with your friend's number to make 9,999) are always somewhere after the number it needs to complement in the sequence, and that you have one more input number than your friend.

So, a seven-number sequence could go:

YFYFYFY (similar to the video) YFFYYFY YFFFYYY FFFYYYY FFYYFYY FFYFYYY...And probably a few other ways I just can't think of right now. What you definitely don't want to happen is something like:

YYYYFFF YYYFFFY...etc. In each of these cases, your friend will have some inputs where you will not have an opportunity to follow-up with a complement.

Mixing things up like this, and running the trick several times in several different ways, will help to keep your friends scratching their heads and thinking you really are a genius!

OMG Is that an HTC One!? Just kidding Awesome trick, I thought up a much simpler version of it (9 instead of 9999) some time ago.

Actually, an oldie, but goodie. I remember amazing my friends with that some 65 years ago.

Is it just me, or is he blowing his covers completely by using the trick together with predictions? Took me no time to figure out what the trick was, because I knew he would be able to predict the result, so I spotted the 9999 sum right away.

Now, if he instead used an extra double row, and just quickly wrote out the result, I think he would convince a lot more people that he's a mathematical genius, because then people will suspect "foul play" a bit less. At least, that's how I think it'll be. Anyone taking this up as a scientific experiment?

It's not just you, no. I was expecting one of those tricks where one tells a person to imagine a number and then performs a series of operations that will always result in a number the person doing the trick knows because the imagined number is again removed from the equation - such as "imagine a number, now multiply it by two, now substract 2, now substract your number, now add 4, now substract your number again -> and now the number in your head is 2!" (Obviously, one should make it a bit more drawn out, heh.)

So with this one, I immediately started to compare the performer's numbers to the ones because it was clear he would do something to negate the randomness of the numbers he was given if he knew them and was able to make additions of his own, and the sums of 9 were quite obvious. The "success rate" of this trick for taking people in could definitely be improved with a changed performance. As it is, I'm sure several people in my social circle would spot it right away.