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| "Math so hard MF wanna derive me" |
We've all heard of the human calculators before. They have been featured on many talk shows and have performed these feats for hundreds of years. Some notable characters you may recognize are Carl Friederich Gauss and Leonhard Euler. Or maybe you won't recognize them. Either way you have probably seen or heard of this before and chucked it up to the fact that they are "geniuses". To which for the most part I would definitely agree. But it turns out that some of these awesome feats are not limited to the naturally talented. Like most intellectual and athletic feats, it can be learned if you have the right method! I like to call this "a chance to make someone think you are a genius". So do you want to impress some people at the bar or at your next holiday party? Join me as we learn to blow people's minds, nerd style.
Magically Multiply By Eleven
Seriously ANYONE can do this one, but hardly no one knows how easy it is. You can easily multiply any two digit number by 11 faster than anyone can even punch the numbers in their calculator much less get it out of their pocket. Lets take 43 for this example. All you do is take the two numbers and add them together and put it in the middle of the two numbers. So for 43x11 the answer is simply 473. It looks like this:
43
x 11
4+3= 7
Answer: 473
That's it! Lets try one that is a little more difficult like 85x11. Since 8+5 is greater than 10 we have the add the 1 to the 8. So the answer is 935. Here is what it looks like:
85
x 11
8+5= 13
Answer: 935
That is about as hard as it gets! After you beat the calculator a couple of times they might challenge you with a 3 digit number. Not to worry! That can be easily done too. Lets take 314. Simply add 3+1 to get 4 and 1+4 to get 5 and put them in the middle to get 3454. That should get your audience nice and warm. The most fun has yet to come.
Squaring Them Fives
Now the earlier stuff might be easy to catch seeing how easy it was, but this is sure to shock them and you will be able to do it with ease. All you need to know is your multiplication tables through 10 by 10. Got it? Alright lets get down. Take a number like 85. Normally you would have no idea how to square (multiply the number by itself) without a calculator, but let me show you how easy it is. All you do is take the first number and multiply it by the next number higher. So that would be 8x9 which is 72. Then just smack a 25 at the end of that. So the answer is 7225. Here is what it looks like:
85
x 85
8 x 9 = 72
5 x 5 = 25
Answer: 7225
I suggest you squint your eyes and hold your head when performing this to give the illusion like you are actually having to do some serious work. Lets take 75. So 7x8 is 56 and then slap the 25 on there to make it 5625. It's really that easy. Why did no one ever teach us this in school? Some things will just never be known.
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| "Dude, stop with this madness! You're gonna make me piss my diaper." |
Hyper Sum
So you've dazzled the audience with your skills so far. Here's a trick that can astonish even the most advanced audiences. You will need something to write with and something to write on for this one. You can use a bar napkin or something like that and whip out the pen you have on hand like a true math nerd. List out 10 blanks on the paper and label them 1 through 10 then have one more blank:
1
2
3
4
5
6
7
8
9
10
Now ask your volunteer to write two random one digit numbers in blanks 1 and 2 without you seeing what they are. Then ask them to sum those numbers and put that in blank 3. Then sum blanks 2 and 3 and put that sum in blank 4 and so on until you reach the end. Lets say our first numbers were 9 and 2. It would look like this:
1 9
2 2
3 11
4 13
5 24
6 37
7 61
8 98
9 159
10 257
Now that this is written have them calculate the sum off all the numbers and memorize it. Now you will tell them that you are going to calculate the sum of all the numbers in your head with only seeing this sheet for 2 seconds. Don't worry! I promise you will be able to do this like a champ.
When they show you the sheet, only memorize the number in blank 7 which in this case is 61. Now remember how easy it was to multiply two digit numbers by 11? Well that is all we need to do here. Multiply 61 by 11 which gives us 671 which is the sum of all ten numbers. You can spout this off in a matter of seconds leaving your audience in complete shock. As they inquire about how you got your skills, have a couple more drinks. If they are becoming doubtful that you are not the mathematical genius that you seem to be, hit them with this last trick.
Drop The Mic
Alright, so it's time to shut down the show. There are different ways to set this trick up but here is the simplest one. Have your volunteer multiply the number 1089 by any three digit number but not to tell you what the number is. Then you ask them how many digits the answer is. Lets say it was 6 digits. So you tell them to call out any 5 digits in any order they like and you will tell them the missing digit. Lets say the answer she had was 278,784 (which is 1089 x 256) and they call out 2, 4, 7, 8, and 8. Now the method behind this trick is that any number that is a multiple of 9 will have digits that sum to a multiple of 9. So 2+4+7+8+8 is 29 which is not a multiple of 9. Since we are missing one number we need to find the next multiple of 9 that is greater than 29 which would be 36. Subtract 29 from 36 to give you 7. Then you say "did you leave out a 7?" to which they will be astonished and you will be the reigning champ of the night! Drop the mic, brah!
Now I must warn you that there is a glitch in this trick. If the numbers they call out happen to add up to a multiple of 9 then you will have no way of determining whether the missing number is a 0 or a 9. So you simply cheat a little. Ask "you didn't leave out a zero, did you?" if they say yes then drop the mic. If not then just say "oh well you didn't leave out a one, two, three, or four, did you? Nor a five, six, seven, or eight. That is because you left out a nine." Glory!!
I mentioned that there are different ways to set up this trick. Let me name off a few:
- Have the volunteer randomly choose a six digit number, scramble its digits, then subtract the smaller six digit number from the larger one. The resulting number will be a multiple of 9. Then continue with the trick to find the missing digit.
- Have the volunteer randomly choose a four digit number, reverse the order of the digits, then subtract the smaller number from the larger. Multiply this number by a three digit number then continue
- Have the volunteer multiply one digit numbers randomly until they produce a number that is 7 digits long. This one is not "guaranteed" to be a multiple of 9 but the chances are very high that they used either a 9, two 3's, two 6's, or a 3 and a six when getting to the seven digit number. Then the resulting number would be a multiple of 9. Continue with the trick as before.
Hope you guys have fun impressing your friends and family with these easy math tricks and maybe even strengthen your math skills at the same time! It turns out that there are a ton more lightning calculation tricks you can learn with a little more practice. Check out this book by Arthur Benjamin called Secrets Of Mental Math: The Mathemagician's Guide To Lightning Calculation And Amazing Math Tricks. This is an amazing book on lightning calculations (it is possible that I have read multiple books on this but I am not admitting anything...) and I strongly suggest that you get it if you are interested in learning more. Check out his TED talk here to see him perform some of these tricks live:
