# is the missing dollar unanswerable **URL:** \ **Category:** Factual Questions\ **Created:** [March 1, 2000, 2:21am UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347 "2000-03-01T02:21:00Z")\ **Posts on this page:** 8\ **Page:** 4
**Author:** ![TalkAboutIslam](https://avatars.discourse-cdn.com/v4/letter/t/4da419/32.png) [@TalkAboutIslam](https://boards.straightdope.com/u/TalkAboutIslam)\ **Post date:** [November 29, 2002, 3:23pm UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/61 "2002-11-29T15:23:51Z")
Sweepkick … you are real ambitious man… lol ---
**Author:** ![Kaitlyn](https://avatars.discourse-cdn.com/v4/letter/k/73ab20/32.png) [@Kaitlyn](https://boards.straightdope.com/u/Kaitlyn)\ **Post date:** [November 29, 2002, 10:54pm UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/62 "2002-11-29T22:54:43Z")
> [@](#): > > \*Originally posted by Wikkit \* > \*\*Yes, you must have, if you’re talking about the post above yours. If a number’s digits are added up, and the sum is evenly divisible by 3, the number is divisble by three. The same is true for 9. > > 30: 3+0=3, evenly divisible by 3. > 60: 6+0=6, evenly divisble by 3. > You can even extend it by adding the sum’s digits. > 23985940303212: 2+3+9+8+5+9+4+0+3+0+3+2+1+2=51, 5+1=6, evenly divisble by 3. > > \*\* True, but there is a difference between the two. Adding the digits for multiples of nine until you have a single digit always reduces to exactly nine. Multiples of three can reduce to 3, 6, or 9. The divisibility formula that’s really freaky is divisibility by 7. Take a multidigit number. Delete the last digit, double it, then subtract the result from the remaining digits. If the result is evenly divisible by 7 , the original number was also. This method sometimes reduces to 0, -7, or -14, but those are evenly divisible by seven, so the rule holds. For example: 4333. Take off the final 3, double it, and subtract that from the remaining digits: 433-6=427. Repeating, take off the seven, double it, and subtract: 42-14=28. You now have a multple of 7, so the original 4333 is a multiple. ---
**Author:** ![TalkAboutIslam](https://avatars.discourse-cdn.com/v4/letter/t/4da419/32.png) [@TalkAboutIslam](https://boards.straightdope.com/u/TalkAboutIslam)\ **Post date:** [November 29, 2002, 11:20pm UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/63 "2002-11-29T23:20:27Z")
wow. you are a mathmatician Number Six ?? ---
**Author:** ![Kaitlyn](https://avatars.discourse-cdn.com/v4/letter/k/73ab20/32.png) [@Kaitlyn](https://boards.straightdope.com/u/Kaitlyn)\ **Post date:** [November 30, 2002, 12:21am UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/64 "2002-11-30T00:21:46Z")
I taught middle school pre-algebra for two years (along with seventh and eighth grade English), but no, I’m not a mathemetician. I actually learned that trick for divisibility by 7 in 5th grade, and it’s stuck with me ever since, just 'cause it seems so cool. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [November 30, 2002, 1:30am UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/65 "2002-11-30T01:30:32Z")
The rule of nines is just a particular case as in base n it will work with (n-1). That rule of divisibility by seven can be proved by proving Mod7(10_n+m) = Mod7(n-2_m) Unfortunately I forgot what I knew of modular math. Can someone do it? ---
**Author:** ![TalkAboutIslam](https://avatars.discourse-cdn.com/v4/letter/t/4da419/32.png) [@TalkAboutIslam](https://boards.straightdope.com/u/TalkAboutIslam)\ **Post date:** [November 30, 2002, 1:47am UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/66 "2002-11-30T01:47:42Z")
> [@](#): > > \*Originally posted by Number Six \* > \*\*I taught middle school pre-algebra for two years (along with seventh and eighth grade English), but no, I’m not a mathemetician. I actually learned that trick for divisibility by 7 in 5th grade, and it’s stuck with me ever since, just 'cause it seems so cool. \*\* it is really so cool for a rule and I am really impressed you still remember that Teaching is one of the best ways of learning something, is not it ??? ---
**Author:** ![Kaitlyn](https://avatars.discourse-cdn.com/v4/letter/k/73ab20/32.png) [@Kaitlyn](https://boards.straightdope.com/u/Kaitlyn)\ **Post date:** [November 30, 2002, 2:07am UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/67 "2002-11-30T02:07:32Z")
Teaching something to another is one of the best ways of reinforcing something you’ve learned, and reinforcing knowledge is one of the steps in learning, so yes. ---
**Author:** ![TalkAboutIslam](https://avatars.discourse-cdn.com/v4/letter/t/4da419/32.png) [@TalkAboutIslam](https://boards.straightdope.com/u/TalkAboutIslam)\ **Post date:** [November 30, 2002, 2:22am UTC](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347/68 "2002-11-30T02:22:26Z")
> [@](#): > > \*Originally posted by Number Six \* > \*\*Teaching something to another is one of the best ways of reinforcing something you’ve learned, and reinforcing knowledge is one of the steps in learning, so yes. \*\* well, well said that is actually what I meant to say, but of course, you have the right expressions… all the difference bt a mother and a non-mother tongue speakers … [Previous page](https://boards.straightdope.com/t/is-the-missing-dollar-unanswerable/8347.md?page=3)