Hi GregMat forum users!

In the explanation for this question, Greg says that we need to add up the digits of the number (5+7+4+2=18, x is not included in the sum) and also uses 18 as an example (1+8=9, which is divisible by 3 therefore 18 is divisible by 3).

I don’t recall learning this from any of the videos of the 2-month study plan. Could someone either point me to the video or explain why this technique works? Thanks!

Hello, for a number to be divisible by 6, it has to be divisible by 2 and 3. Since it is already given that the number ends with 2, it is divisible by 2. To be divisible by 3, sum all the digits of that number and the resulting number should be divisible by 3 for the whole number to be divisible by 3.

These are the divisibility rules of numbers. We have the same for 4,5,6,8,9,10,11.

I don’t remember where they might be in Greg’s videos since it has been a long time since I’ve seen them. But I would check out the quant concepts series, and in the Integer section, you might find them. If not, if you go to Recommended Resources - GregMat and scroll down, you’ll see a math cheat sheet from targettestprep which contains divisibility rules.

Also, Prepswift(Greg’s other project) has videos for the same. https://www.prepswift.com/gre-quant.

Good Luck!

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Thanks! I looked at the Target Test Prep divisibility rules and then I also googled and found the Wikipedia page for it: Divisibility rule

I thought that there was some kind of technique that we had to apply, but as far as I can tell we’re just supposed to memorize the rules and use them as shortcuts.