Latest episode! Look Like A Mathlete!

[lonedragon]

Hi all!

came across the newewst episode on scam school. i believed you guys have already watched it. i've already did this trick before the show broadcasted it. feel free to post ur comments here! curious on how this simple effect has created a magical impact on you and your friends.. =)

[lonedragon]

why not we shall contribute some tricks involving mathematical concepts behind it and let the thread cone alive? i shall start by contributing one.

mine uses a calculator. so if u want to try it out and learn u can go get one.


right let's start.


well, in a year there's 12 months and there's around 30 days in a year. in these 30 days, pick any 3 consecutive days and add them up using the calculator.

then add 27 to the total, thn mutiply by 67 to the new total. that's the grand total.


so from here how do you guess the 3 numbers we 1st picked?

u take the last two digits o the grand total and minus 10 and u get the 1st digit! then u'll know the rest of the numbers (they are consecutive). it's that easy!

so now let's listen to yours!

[evadukar]

Pick:
9
9
18
27
45
72
117 <-- kills the "two-digit * 11"-thing

sure, he can still do 117*11 in his head, but this might just give you the time you need to do this on the calculator ^^

[evadukar]

ok, turns out this works with 6,9 7,9 8,8 8,9 9,7 9,8 and 9,9

code:
for (int i = 1; i < 10; i++) {
 for (int j = 1; j < 10; j++) {
  int x = j, y = i + j, z = 0;
  for (int k = 0; k < 7; k++) {
   z = y;
   y = y + x;
   x = z;
   if (k < 4 && y >= 100) System.out.println(i + "," + j);
  }
 }
}

[loomis]

It's almost as easy to multiply a 3 digit number by 11 as it is for a 2 digit number.

For example, 117* 11 = 1 (1+1) (1+7) 7 = 1287

[greymatters]

You can take this week's scam s atep further, and determined the answer to the 10th number divided by the 9th number in your head, as well.

I cover this in my Scam School Mathemagic blog post, as well as carious ways of using the Fibonacci idea to scam people in Nim games.

[aofl]

Multiplying a number of any length by 11 is easy. Here's how:

Say you have a 4 digit number, abcd. To multiply this by 11, do this:

1) Write the last digit down. This will be the last digit of the answer.

2) In front of the digit you just wrote down write the sum of the digit in the original number you just looked at and the digit to the immediate left of it. If the sum is greater than 9 then just write the last digit and remember to carry the 1.

4) Continue to the left repeating step 2 until you get to the first digit of the original number. Remember that there is an implied 0 at the beginning of the number so you have to add the first digit to that along with the carry.

For example: 3489756 * 11:
Write down 6:
_ _ _ _ _ _ _ 6
Add 6+5, write down the 1 and remember the carry.
_ _ _ _ _ _ _ 16
Add 5 + 7, add the carry, and remember the carry.
_ _ _ _ _ _ 316
Add 7+9, add the carry, and remember the carry.
_ _ _ _ _ _ 7316

And so on to get the final answer (I didn't use a calculator to figure this out or check it):
38387316

All of this means that you don't have to limit yourself to small starting numbers. You can have them pick 3 or more digit numbers. This will have them take more time doing the calculations and since multiplying by 11 is easy you have a better chance of beating them. Of course if the numbers are really long you'll probably want to write it down.

A

[fatpenguins]

EDIT Sorry greymatter didn't see you had the same explanation up when I wrote mine. Nice blog post!

Its not that hard to figure out why this works

It will work even with non single digit numbers as well.

Say their random numbers are a and b This is what your prey's table of numbers will be (in variable form of course)

#1= a
#2= b
#3=#1+#2= a+ b
#4=#2+#3= a+ 2b
#5=#3+#4= 2a+ 3b
#6=#4+#5= 3a+ 5b
#7=#5+#6= 5a+ 8b
#8=#6+#7= 8a+13b
#9=#7+#8=13a+21b
10=#8+#9=21a+34b

If you then add all those components up and add like terms you get that adding 1 through 10 gets you

55a+88b

so you could either do this in your head or like the scam says look at number 7. if you multiply number 7 (which is 5a+8b) by 11 you get 55a+88b just the same as your final answer.

So this will work for any initial numbers they choose its just easier to multiply by 11 with smaller numbers I suppose.

As others have also said its easy to multiply by 11 just take the number and add it to the number with a 0 behind it (ten times the number)

so 5438 for example times 11 is just 5438+54380=59818