Math Trick

[kathydwells]

Ok some of you math wizards tell me how they figured this out.

Here is a math trick so unbelievable that it will stump you. Personally I would like to know who came up with this and why that person is not running the country.

1. Grab a calculator. (you won't be able to dothis one in your head)
2. Key in the first three digits of your phone number (NOT the area code)
3. Multiply by 80
4. Add 1
5. Multiply by 250
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Subtract 250
9. Divide number by 2


Do you recognize the answer???????

 

[dbuck]

You lost me at step 3.

 

[Beach Runner]

Let x=the first 3 digits of your phone number and y=the last 4 digits of your phone number.
(250(80x+1)+y+y))/2=
(20000x+250+2y-250)/2=
(20000x+2y)/2=
10000x+y

Let's say your number is 231-6789. Multiplying the first three digits by 10000 merely moves the 231 to the 10^6 through 10^4 places to give 2310000. Then adding the last four digits (say 6789) gives you your correct number, 2316789. It could easily work with other numbers.

There are many variations, such as:
1. Grab a calculator or launch Excel
2. Key in the first three digits of your phone number (NOT the area code)
3. Multiply by 50
4. Add 1
5. Multiply by 400
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again
8. Subtract 400
9. Divide number by 2

It's just a matter of simple algebra. I give these little brain teasers in a math class I teach to make them think and prove why the solution is true.

 

[Cil]

This reminds me of Chocolate Math.
This "trick" worked for me, though I don't understand how. (I was always terrible at math.)

CHOCOLATE MATH
> >
> > YOUR AGE BY CHOCOLATE MATH
> >
> > This is pretty neat.
> >
> > DON'T CHEAT BY SCROLLING DOWN FIRST!
> > It takes less than a minute....... Work this out as you read, you
will
> > need a calculator.
> >
> > Be sure you don't read the bottom until you've worked it out!
> > This not one of those waste of time things, it's fun.
> >
> >
> >
> > 1. First of all, pick the number of times a week that you would
> > like to have
> > chocolate. (more than once but less than 10)
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > 2. Multiply this number by 2 (Just to be bold)
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > 3. Add 5. (for Sunday)
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > 4. Multiply it by 50 I'll wait while you get the
> > calculator................
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > 5. If you have already had your birthday this year add
1755....
> >
> >
> >
> >
> >
> >
> >
> > If you haven't, add 1754 .
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > 6. Now subtract the four digit year that you were born.
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > You should have a three digit number
> >
> >
> >
> >
> >
> >
> >
> > The first digit of this was your original number (i.e., how many
> > times you want to have chocolate each week).
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > The next two numbers are .
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > YOUR AGE! (Oh YES, it is!!!!!)
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > THIS IS THE ONLY YEAR (2005) IT WILL EVER ! WORK, SO SPREAD IT
>AROUND
> >

 

[Jdarg]

Thanks y'all--nothing like using my ol' calculator to get my brain working in the AM! Now where is my coffee?!

 

[kathydwells]

BR, I knew that you would be the one to set me straight on this one. Good job. I don't do math. My brain just can't compute. I am good with a calculator though, thank God for them, or no telling what my check book would look like. Happy New Year!!!!

 

[Beach Runner]

BR, I knew that you would be the one to set me straight on this one. Good job. I don't do math. My brain just can't compute. I am good with a calculator though, thank God for them, or no telling what my check book would look like. Happy New Year!!!!

I wish my textbook wasn't at my office. It has tons of cute math puzzles in it. The purpose is to try to get students who think they don't like math to see how interesting it can be.