Can I Get Your Digits?

February 23, 2015 at 10:33 pm 6 comments

Saw this on a t-shirt recently:

PIN Digits Pi
It made me think about this problem involving digits.

Consider the number

1234567891011121314151617181920

obtained by writing the numbers from 1 to 20 in order side-by-side.

What’s the greatest number that can be obtained by crossing out 20 digits?

If a fetching lady or handsome gent catches your fancy by solving that problem, you might want to ask her or him…

How can I know so many digits of π and so few digits of your phone number?

And if he or she still hasn’t taken leave of you, then you could really press your luck with the following:

  1. Ask your new friend to write down a number with four or more digits.

459,163

  1. Then, have your friend add the digits.

4 + 5 + 9 + 1 + 6 + 3 = 28

  1. Subtract the sum from the original number.

459,163 – 28 = 459,135

  1. Have your friend cross out one of the digits, and then read the remaining number aloud to you.

45,935

  1. Then, miraculously announce the missing digit.

1

 

The secret to the trick? Simple. Just add the digits of the number that your friend reads aloud, and then figure out what number must be added to get the sum to a multiple of 9. Above, the digits of the number 45,935 have a sum of 26, which is 1 less than a multiple of 9, so the removed digit is 1.

 

Entry filed under: Uncategorized. Tags: , , , , .

And the Oscar Goes To… a Mathematician? Deliver Us Not Into Bad Math

6 Comments Add your own

  • 1. Sam  |  February 23, 2015 at 11:52 pm

    Awesome! Would the first one be 91234567892 ?

    Reply
    • 2. venneblock  |  March 1, 2015 at 4:36 pm

      Sam, I don’t make it a practice to give away answers. Sara posted a reply that — if correct — would be greater than your answer.

      Reply
  • 3. Sara  |  February 24, 2015 at 3:22 pm

    I can get 95617181920

    Reply
  • 4. zachdcox  |  February 28, 2015 at 6:24 pm

    I posted the steps on my FB page and invited my friends to post their digits and I would ‘magically’ name the missing digit. (And I posted a link to this post with the prefix SPOILER ALERT).

    And on G+ I added – ‘Homework – Post A Proof’

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Trackback this post  |  Subscribe to the comments via RSS Feed


About MJ4MF

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

MJ4MF (offline version)

Math Jokes 4 Mathy Folks is available from Amazon, Borders, Barnes & Noble, NCTM, Robert D. Reed Publishers, and other purveyors of exceptional literature.

Past Posts

February 2015
M T W T F S S
« Jan   Mar »
 1
2345678
9101112131415
16171819202122
232425262728  

Enter your email address to subscribe to the MJ4MF blog and receive new posts via email.

Join 232 other followers

Visitor Locations

free counters

%d bloggers like this: