# Cool Mathematical Trick

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#### hotwaterwizard

##### Full Member level 3 How does this work?

Math Problem

1. Grab a calculator. (you won't be able to do this one in your head)
2. Key in the first three digits of your phone number (NOT the area code)
3. Multiply by 80
5. Multiply by 250
8. Subtract 250
9. Divide number by 2

#### Kral Re: Math Problem?

hotwaterwizard,
.
Let the 1st 3 digits = A
Let the last 4 digits = B
Let C = result of step 5

step 5: C = (A X 20000) + 250 because 80 X 250 = 20000.
This is equivalent to moving the 1st 3 digits to the left 4 places,multiplying by 2, and adding 250 to the result.

Step 8: this is equivalent to C + 2B -250 = 2 X (1st 3 digits shifted left 4 places) + (2 X last 4 digits)
Step 9: This is equivalent to (1st 3 digits shifted left 4 places) + last 4 digits.

#### hotwaterwizard

##### Full Member level 3 Math Problem?

All greek to me but the way you explain it makes sense to me somehow.

#### Kral Re: Math Problem?

hotwaterwizard,
Perhaps its the "shifting" that needs some clarification. Grab a piece of quadrille papaer. Use 7 of the squares. Label the squares, from left to right, 1,2,3,4,5,6,7.
Write the first 3 digits in squares 5,6,7. Multiply the number by 10,000. The 3 digit number now appears in squares 1,2,3. Squares 4,5,6,7 now contain zeros. In the problem, the 3 digit number is multiplied by 20,000 in 2 steps: multiply by 80, and then multiply by 250. So the result in squares 1,2,3 is twice the value of the 1st 3 digits. This value gets divided by 2 in the last step, resulting in the original 3 digt number in squares 1,2,3.
Regards,
Kral

#### hotwaterwizard

##### Full Member level 3 Math Problem?

Got it they just play with the numbers and shift the decimal point around in a confusing sequence of events.

#### Kral Re: Math Problem?

hotwaterwizard,
Yup

#### Naveed Alam

##### Full Member level 5 Re: Math Problem?

Kral,
u r intelligent but see it like this.
let phone no. is 2612338

so, {(261*80)+1}*250 = 5220250
now,
5220250 + 2338 + 2338 -250=5224676
5224676/2 = 2612338

So, got ur phone number again..

enjoy,
Naveed

#### noorm

##### Newbie level 5 Math Problem?

Take first four digits of your phone number
multiply by 25
multiply by 200
add 5 times of last tree digits
subtact 1000
divide by 5
Check the result

therefore this works so

Thanks

#### eagle_zh

##### Newbie level 2 Math Problem?

Consume phone number as x*10000+y;
so
3.multiply the first three digits by 80

80*x;

80*x+1

5. Multiply by 250

(80*x+1)*250

(80*x+1)*250+y

(80*x+1)*250+2*y

8. Subtract 250

(80*x)*250+2*y

9. Divide number by 2

10000*x+y

the result is just what you input.

#### electricpete

##### Member level 2 Re: Math Problem?

It's a pretty cool math trick. My 8 year old daugher is very amazed by things like this (and I have a hard time impressing her with much any more).

It's no fun to tell it to an engineer, they just analyse to figure out how it works

#### bangash

##### Member level 5 Math Problem?

Its just shifting your first three nos and then adding the last 4 digits but still cool. Many people wont find it that easy

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