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Here's some stupid math tricks:

1. Pick any number.
3. Multiply your result by 10.
5. Subtract 10.
6. Divide by 10.
8. Divide by your original number.
9. Subtract 3 from the number

The result is always 0 (zero).

Magic Number

1. Pick a number between 1 and 9.
2. Multiply this number by 2.
4. Multiply by 50.
6. Last step: Subtract the four digit year that you were born.

The first digit of the answer is the original number you chose. The remaining two digits represent your age.

*: given for the year 2000. Increase these values by one for each successive year.

1. Choose any prime number greater than 3
2. Square that number
4. Divide by 12

The remainder will always be 3.

1. Ask the person to multiply the first number of his or her age by 5.
2. Tell them to add 3.
3. Now tell them to double this figure.
4. Finally, have the person add the second number of his or her age to the figure and have them tell you their answer.
5. Deduct 6 and you will have their age.
I guess I'm kinda stuck with the "stupid" title, but my trick is far from stupid. This one is really neat when you've done it enough to be able to do it in your head. Here's how it's done.

Have your friend or whomever you're doing the trick for think of a quadratic expression (i.e. 5x2+2x-8). Then, have them give you the value for the expression when x equals 0, 1, and 2. Here's how you can find their expression on paper:

Write the answers in a row (In our example: -8 -1 16). Subtract the first number from the second and the second from the third. Write these two answers in a second row. Then, subtract the second number from the first and write your answer in a third row. You should end up with something like this:

-8   -1   16
7   17
10

The coefficient for the quadratic term is half of the last value in your pyramid (5). To get the coefficient of the linear term, subtract half of your last number from the first number in the second row (7-5=2). The constant value is simply the first number in the first row.

Now, you can be all like, "It's 5x2+2x-8, duh". Then they'll be all like, "Dude, you are way smart."

Note: This works best on high-school kids.

Number Transformation II:

1. Pick any number between 1 and 15;
2. multiply that number by 2;
3. add 3 to the result;
4. double the result;
5. subtract 10 from the result;
6. multiply the result by 10;
7. divide the result by 2;
8. add 6 to the result;
9. divide the result by 2;
10. multiply the result by 5;
11. multiply your original number by 2, and add it to the result;
12. add 1 to the result;
13. divide the result by 2;
14. square the result;
15. add 25 to the result;
16. square your original number, and subtract it from the result;
17. add 100 to the result;
18. divide the result by 7;
19. take the third root of the result; and, finally,
20. multiply the result by 0.
The answer will always be zero.

Here is a trick I learned as a child which I think is worth documenting for eternity in cyberspace. It is how to multiply the numbers six through ten using only your fingers.

First assign numbers in the following way to the fingers of both hands

1. pinky = 6
2. ring finger = 7
3. middle finger = 8
4. index finger = 9
5. thumb = 10

Fine, you have just constructed a simple 'digital' computer.

Now keep your hands in front of you, palms facing you, thumbs up.

Let's say we want to multiply 6 x 7.

Touch and hold the pinky of the left hand (6) to the ring finger of the right hand (7). Now with your hands in this position, fingers touching, hands in front of you, palms facing you and thumbs up:

1. count any touching fingers as ten each. (10 for left pinky plus 10 for right ring finger = 20)
2. count any fingers below the touching fingers as ten each.(ten for the right pinky which is below the touching fingers = 10)
3. multiply the number of fingers above the touching fingers on the left hand by the number of fingers above the touching fingers on the right hand. (there are four non-touching fingers above the touching fingers on the left hand and three on the right so 4 x 3 = 12).

So now add the numbers obtained from steps 1, 2, and 3. You get 20 + 10 + 12 = 42 and of course the original problem 6 x 7 = 42

Another example? Let's try an easy one. 7 x 8. Hands in position. Touch and hold the left ring (or 7) finger to the right middle (or 8 finger)

1. count the two touching fingers as 10 each = 20.
2. count any non-touching fingers below the two touching fingers as ten each (in this case the left pinky plus the right pinky and ring total up to = 30).
3. count up the non-touching fingers above the touching fingers on the left hand and the non-touching fingers above the touching fingers on the right hand (in this case there are three on the left and two on the right) and multiply them. 3 x 2 = 6

So now add the numbers obtained from steps 1, 2, and 3. 20 + 30 + 6 = 56 and the original problem was 7 x 8 and the answer of course proves to be 56.

as ariels points out, see Russian peasant multiplication and also see mathematics

I'm not sure if its kosher to simply submit another person's material, but Eugene Mirman has a great technique for visualizing just how large one million really is:

"Imagine a box with 1,000 spoons. Now double that. Again. Again. Again. Again. Again. Again. Again. Again. Again. Now subtract 24,000 spoons, and you have a bit of an idea.

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