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#1 2008-11-26 16:46:40

Jai Ganesh
Administrator
Registered: 2005-06-28
Posts: 45,966

A simple mathematical trick.

Ask your friend to think of any natural number. Then ask him to multiply the number by 2 and then add the result by 2. Then ask him to multiply the result by 5 and then add 5 to the result. Ask him to reveal what answer he finally got. You'd be able to say what number he had in his mind at the beginning in less than 2 seconds!!!

How it is done:- Whatever be the result, the units place would always be 5. Omit the 5 and subtract 1 from the remaining part of the result and thats the number your friend had in mind initially!!!

Example : Lets say your friend starts with 15.
15x2=30, 30+2=32
32x5=160, 160+5=165.
When he says 165 is the result, omit 5 in the units place, you are left with 16. Subtract 1 from 16, you get 15! As simple as that!

PS:- 'him' is interchangeable with 'her' wherever necessary smile


It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#2 2008-12-02 15:12:30

LuisRodg
Real Member
Registered: 2007-10-23
Posts: 322

Re: A simple mathematical trick.

Very nice.

Do you have any more similar to this one?

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#3 2008-12-02 17:31:56

Jai Ganesh
Administrator
Registered: 2005-06-28
Posts: 45,966

Re: A simple mathematical trick.

Luisrodg,
Thanks!
This is a lengthier one though!

I have given the names of various cities. Ask your friend to choose his/her favorite city.

Paris          Berlin          Rome          Chicago          Wellington
Sydney      Hongkong    Bangkok      Tokyo             Moscow
Kolkata      Kuwait         Singapore    Lisbon            Cape Town

Now all you have to do is ask him/her to tell in which column or columns A, B, C, and D the name of his/her favorite city appears.

     A                                B                              C                            D
Singapore                     Moscow                      Lisbon                    Tokyo
Paris                            Cape Town                 Chicago                  Moscow
Rome                          Berlin                         Kuwait                    Kolkata
Kolkata                        Sydney                      Singapore               Bangkok
Cape Town                  Rome                         Sydney                   Singapore
Wellington                   Kolkata                       Wellington               Kuwait
Tokyo                         Moscow                       Hongkong              Cape Town
Hongkong                   Hongkong                    Kuwait                   Lisbon


The trick : how do you tell the name of the city within 5 seconds?
Assign values A=1, B=2, C=4, D=8. In whichever column the name of the city appears, add the value. Go to the top. The cities are numbered as 1,2,3,4,5 in the first row, 6,7,8,9,10 in the second and 11, 12,13,14,15 in the third.

assuming your friend says the name of the city he/she had in mind is in columns A, B and D, quickly add 1,2,8; you get 11. Hence the city is Kolkata.

I hope you like this one as it involves numbers. I am sure you would impress your friend(s). The names of cities can be replaced by celebrities/sports events etc.

I shall be posting a mathematical trick (which I had already posted long ago) very soon!


It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#4 2008-12-02 20:32:29

Jai Ganesh
Administrator
Registered: 2005-06-28
Posts: 45,966

Re: A simple mathematical trick.

LuisRodg,
Here's the other one I said I'd tell you.

Ask your friend to think of any number containing any number of digits (more than 1 digit). At no point of time, he/she has to tell you of the number until you ask fot it in the final step.
Assume your friend has the number 8765 in his/her mind.

Then ask your friend to add all the digits of the number he/she had thought of.
That is, the person would have to add 8, 7, 6, and 5 (Total=26).

Then ask your friend to subtract this sum from the original number.
That is, he/she would have to subtract 26 from 8765. The result would be 8739.

Finally, ask your friend to hide any one of the digits from the final result and reveal the other ones. The only condition being 0 should never be hidden.
Assume, the person hides 7, and reveals the other digits, viz.8,3, and 9.

The trick : You'd be able to tell the number hidden within 10-15 seconds. this is how it is done.

Add the numbers the person tells you. In the illustration, add the numbers 8,3, and 9. You get 20. All you got to do is find the next higher multiple of 9 and find the difference. that would be the number your friend had hidden.In this case, 27-20=7. This always works provided the person doesn't make any calculation mistake or you do in haste. If the total of the numbers he/she reveals after hiding a number is a multiple of 9 in itself, the number hidden is 9. Always remember to forewarn your friend not to hide a zero.

PS: Please don't let the trick be known to everyone to whom you demonstrate the trick.


It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#5 2008-12-03 01:13:40

LuisRodg
Real Member
Registered: 2007-10-23
Posts: 322

Re: A simple mathematical trick.

The last one is VERY nice!

Thanks ganesh!

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#6 2008-12-17 15:02:19

careless25
Real Member
Registered: 2008-07-24
Posts: 560

Re: A simple mathematical trick.

Heres some more:

1)
Ask you friend to think of a number.
Add 9 to this number.
Double the answer.
Add 3 to it.
Now multiply by 3.
Subtract 3 from the answer.
Divide by 6.
Subtract by original number.

The answer will always be 10
You can limit the friends choice by giving him only 1-100 to pick from, which will make calculation easier for him.

2)
Ask your friend to think of a number.
Multiply by 5.
Add 25 to it.
Divide it by 5.
Take the quotient and subtract it by original number.
Multiply by 3.

The answer will always be 15

3) A trick using the same concept in Ganeshs 2nd trick.

Create a table like this:

1   2   4   8   16   32
3   3   5   9   17   33
5   6   6   10  18  34
7   7   7   11  19  35
9   10 12  12  20 36

continue all the way to 63...
first column is all odd numbers
second column follows this pattern: +1 then +3, +1 then +3, etc...
Third column follows this pattern: +1, +1, +1,then +5, +1, +1, +1, then +5 etc...
Fourth column follows this pattern +1 * 7 then +9, +1 *7 then +9, etc...
Fifth : +1 * 15, then +19, +1 * 15, then +19 etc...
Sixth column : go all the way from 32 to 49 then skip 50 and go until 63.

After you have this table ready ask your friend/relative or stranger to point to the column his age shows up in.
Add up the numbers at the top of the columns indicated and the sum will be equal to his/her age.

For example if your friend is 12 years old.
He points to the 3rd and 4th column. You add up 4(top number of 3rd column) and 8(top No. of 4th Column) and you get 12.


Have fun!

C25

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