Why is Today “Prime Day”?

July 11, 2017 at 12:21 pm 7 comments

Today is July 11, which the marketing folks at Amazon* have dubbed “Prime Day.” They’ve even created a spiffy, little banner image for it:

PrimeDay.png

Ooh… pretty!

The selection of 7/11 as Prime Day was no doubt deliberate, since both 7 and 11 are prime numbers, though one has to wonder why Amazon ignored the other 52 (or 53, if it’s a leap year) dates they could have chosen:

  • February
    • 2/2
    • 2/3
    • 2/5
    • 2/7
    • 2/11
    • 2/13
    • 2/17
    • 2/19
    • 2/23
    • 2/29 (some years)
  • March
    • 3/2
    • 3/3
    • 3/5
    • 3/7
    • 3/11
    • 3/13
    • 3/17
    • 3/19
    • 3/23
    • 3/29
    • 3/31
  • May
    • 5/2
    • 5/3
    • 5/5
    • 5/7
    • 5/11
    • 5/13
    • 5/17
    • 5/19
    • 5/23
    • 5/29
    • 5/31
  • July
    • 7/2
    • 7/3
    • 7/5
    • 7/7
    • 7/11
    • 7/13
    • 7/17
    • 7/19
    • 7/23
    • 7/29
    • 7/31
  • November
    • 11/2
    • 11/3
    • 11/5
    • 11/7
    • 11/11
    • 11/13
    • 11/17
    • 11/19
    • 11/23
    • 11/29

One of my favorite problems is based on the numbers 7 and 11. Here’s a modified version of it, tailored to Amazon’s special day:

An online shopper placed four items in his cart. When he checked out, his credit card was charged $7.11. Shortly thereafter, a programmer realized there was an error in the code, and total price had been calculated by multiplying the prices of the four items. The customer service department was about to alert the customer to the error, but the programmer informed them that the total price would have still been $7.11 if the prices had been added. No harm, no foul.

There was no sales tax. What was the cost of each item?

Good luck! Happy shopping!

* No, Amazon did not pay me to write a blog post about Prime Day.

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7 Comments Add your own

  • 1. Chris Meador  |  July 11, 2017 at 8:40 pm

    I bet a good way to start solving your favorite problem would be to multiply 7.11 by 10^4 to get us into the world of whole numbers.

    Reply
    • 2. venneblock  |  July 13, 2017 at 6:45 am

      Wouldn’t multiplying by 10^2 be sufficient?

      Reply
      • 3. Chris Meador  |  July 17, 2017 at 10:21 am

        If w*x*y*z = 7.11 where each of w,x,y,z might be to two decimal places, we need to multiply all four of them by 10^2 to guarantee whole numbers on the left side of the equation — which means multiplying the right side by 10^8 for balance. As a result, the prime factorization of 7.11 * 10^8 is 2^6 * 3^2 * 5^6 * 79.

        With 15 prime factors, that gives us 4^15 ways to assign them to w,x,y,z, and that’s a big number (over one billion). It’s too many for me to do by hand so I asked my computer to test them. Trusty rusty computer only had to try ~5.8 million different arrangements before landing on:

        w = 2^3 * 3 * 5 = 120
        x = 2^2 * 79 = 316
        y = 2 * 3 * 5^2 = 150
        z = 5^3 = 125

        So, the four prices are $1.20, $3.16, $1.50, and $1.25.

        Is there a better way to solve this problem?

  • 4. Xida Ren  |  July 11, 2017 at 9:22 pm

    We’re looking for four numbers that add to 7.11 and multiply to the same. Looking for these numbers in the reals would give us infinitely many answers. Maybe the key lies in the fact that prices are always whole multiples of 0.01 (i.e., in a whole number of cents).

    Reply
    • 5. venneblock  |  July 13, 2017 at 6:49 am

      Yeah, you definitely need to consider the real-world aspect of this one to find the (unique) solution.

      Go Griffins!

      Reply
  • 6. Marjan  |  July 13, 2017 at 8:55 pm

    🙂 love this blog. I’ve got a few kids working on it…

    Reply
    • 7. venneblock  |  July 14, 2017 at 12:41 pm

      I bet I can guess which kids! Good luck to them!

      Reply

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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.

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