Can you pick the numbers that are divisible by...?

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Divisibility Minefield Quiz

by puckett86

Created Apr 20, 2012 in Science
Featured Oct 4, 2012
Game Plays 47,252

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	MisterF:	Oct 4th, 2012 at 04:27 GMT		12 points
	Trick for 7s: Remove the last digit of the number, multiply by 2, and subtract that number from the rest of the original number. Repeat until you get a number that you know is/isn't divisible. Example: 9254 -> 925-24=917 -> 91-27=77... etc.

	CPADave71:	Oct 4th, 2012 at 05:04 GMT		24 points
	Or you could just try long division in your head...

	Arcylix:	Oct 4th, 2012 at 07:52 GMT		9 points
	Really, it's quite easy. The big numbers look scary, but you just work with smaller numbers. For instance, does 8 go into 38,231 evenly? Nearest multiple to 38,000 is 32,000. Now you have 6,231 to deal with. Nearest one to that is 5,600. Now you have 631 to deal with. Easier to divide 631/8 instead of 38,231.

	buJ:	Oct 4th, 2012 at 09:11 GMT		3 points
	A few methods that work: For 3 , add all the numbers and they should be divisible by 3. For 6, divide by 2, then use the trick for 3. For 4 divide by 2 and it should be even. 8, divide by 2 twice and it should be even. Problem was I kept trying to find 'tricks' to simplify things I ended up not having enough time to finish :D

	Ulkomaalainen:	Oct 4th, 2012 at 09:15 GMT		1 point
	Precisely - it's not really full long division - you can forget your results up to there, so you don't have to carry any baggage. Heads up for the 7's trick, I didn't know that one - and I'm a maths teacher. Goes to show why pupils are so bad there if teachers don't know their stuff...

	RockMontage:	Oct 4th, 2012 at 09:25 GMT		1 point
	I just remembered how to do long division!

	Tony_A:	Oct 4th, 2012 at 12:22 GMT		14 points
	@Arcylix, your example is even easier when you remember that you can't multiply an even number to any other number, even or odd, and get an odd number as a result.

	Pogues:	Oct 4th, 2012 at 12:28 GMT		11 points
	MisterF: Is the fact your method is called a "trick" to trick others into doing a method more complicated than just doing the long division?

	ElixirOfRivalry:	Oct 4th, 2012 at 13:24 GMT		3 points
	Wow, first time I've ever done a quiz first try (without cheating) only 16 seconds left though. Good Quiz!

	TheHYPO:	Oct 4th, 2012 at 13:35 GMT		0 points
	I have never heard of MisterF's trick. It may be "slower" than long division, but I can't do long division in my head. I can do that 7s trick in my head. I like it. I also used Arcylix's method for many of them because I forgot that for numbers like 8 you can divide down to an even/odd test

	sub:	Oct 4th, 2012 at 13:42 GMT		3 points
	Arcylix: For 8s, you just need to look at the last 3 digits of the number (because 1000 is divisible by 8). Then, you either divide by 8 directly, or you divide by 4 then by 2. For example, take 1751875436 -> 436 -> 436/4=109 which is not divisible by 2.

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	erak:	Oct 4th, 2012 at 14:10 GMT		1 point
	Argh! Ran out of time a split second before clicking the final correct answer.

	david6k:	Oct 4th, 2012 at 14:41 GMT		3 points
	@sub - 8s are easier than that - eights go into 200 and into 80. So 1751875436 -> 436 -> 36 -> No. 97936 -> 936 -> 136 -> (subtract 80) -> 56 -> Yes.

	Tarbosh:	Oct 4th, 2012 at 15:44 GMT		1 point
	I basically used the trick Arcylix used. Although, from his example I'd like to think people would be able to identify that 8 won't go into an odd number :-).

	caramba:	Oct 5th, 2012 at 16:01 GMT		1 point
	The trick for 9s is really easy. Add up all the digits. If you can divide 9 into the sum without a remainder, then the number is also divisible by 9.

	ktappe:	Oct 7th, 2012 at 23:07 GMT		1 point
	@Pogues: Long division is not easy unless you have paper. Any trick that will speed things up and keep me from getting out a pen & paper is appreciated.

	ktappe:	Oct 7th, 2012 at 23:10 GMT		1 point
	BTW, downvotes to the quiz for not showing the correct answers when the quiz ends. I only got 1/2 of them and wanted to see which one I missed that 51% of others got, but all it listed was a "?".

	wiggytitch:	Oct 8th, 2012 at 12:40 GMT		2 points
	@ktappe; It's no fault of the quiz that it doesn't show the answers if you get them wrong, it's the format that doesn't allow it.

	BillyJoelRulez:	Nov 5th, 2012 at 02:34 GMT		4 points
	I can't believe I got all correct except 2 going into 14. Facepalm.

	featherfish81:	Jan 1st, 2013 at 23:14 GMT		1 point
	Just to clarify, for the ones without another trick (like 7), you don't have to do the full long division. It doesn't matter what the answer is, so just carry over the remainder and see if there's one left at the end. (IE, 7 into 90,846...7 into 9 leaves 2, 7 into 20 leaves 6, 7 into 68 leaves 5, 7 into 54 leaves 5, 7 into 56 goes evenly)

	Talikiasmi:	Feb 13th, 2013 at 04:06 GMT		1 point
	Only 58% got that 5 goes into 10... wow. Just... wow.

	kadorwm:	Mar 27th, 2013 at 05:31 GMT		2 points
	featherfish, you are aware that what you described when saying you don't need to do long division is long division... right?