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Intern  Joined: 29 Sep 2013
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If a, b and c are integers, is abc divisible by 4?  [#permalink]

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Question Stats: 68% (02:10) correct 32% (02:04) wrong based on 225 sessions

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If a, b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even.
(2) a + 2b + c is odd.

Hi Everyone,

Not sure if this has been posted before, but here is my question: I'm using the manhattan gmat advanced quant book (just because I tried doing a GMAT practice test and found that I wasn't doing as well as I hoped on the quant - 70th percentile) and I found the suggestion to use scenario charts very interesting. However, when I try using them, I can't get my time to go below 3:30s to 4 mins which I know is too long to be working on any one problem.
I'm sure people have had success with the tool, which is why MGMAT recommends it; however, does anyone have any idea on how to use the chart more efficiently?

As an example:
I'm using the chart for try-it problem 4-7 in the MGMAT advanced quant book:
If a,b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even.
(2) a + 2b + c is odd.

I have 5 columns: a, b, c, 4th column: to ensure a+b+2c is even and 5th column to check if abc is divisible by 4. I try different values for a, b and c and etc till I find whether abc is always divisible by 4 or not.

MGMAT suggests to look at a, b and c as O/E numbers. I find this would not have been intuitive for me (since the question asks "is it divisible by 4," I would automatically start by trying out numbers.)

Anyways, if anyone has some tips or something that you think I should do different, please feel free to share. Any help will be appreciated!

Thanks!!

Originally posted by dukenukem on 29 Sep 2013, 19:14.
Last edited by Bunuel on 29 Sep 2013, 23:58, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
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Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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dukenukem wrote:
If a, b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even.
(2) a + 2b + c is odd.

Hi Everyone,

Not sure if this has been posted before, but here is my question: I'm using the manhattan gmat advanced quant book (just because I tried doing a GMAT practice test and found that I wasn't doing as well as I hoped on the quant - 70th percentile) and I found the suggestion to use scenario charts very interesting. However, when I try using them, I can't get my time to go below 3:30s to 4 mins which I know is too long to be working on any one problem.
I'm sure people have had success with the tool, which is why MGMAT recommends it; however, does anyone have any idea on how to use the chart more efficiently?

As an example:
I'm using the chart for try-it problem 4-7 in the MGMAT advanced quant book:
If a,b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even.
(2) a + 2b + c is odd.

I have 5 columns: a, b, c, 4th column: to ensure a+b+2c is even and 5th column to check if abc is divisible by 4. I try different values for a, b and c and etc till I find whether abc is always divisible by 4 or not.

MGMAT suggests to look at a, b and c as O/E numbers. I find this would not have been intuitive for me (since the question asks "is it divisible by 4," I would automatically start by trying out numbers.)

Anyways, if anyone has some tips or something that you think I should do different, please feel free to share. Any help will be appreciated!

Thanks!!

If a, b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even --> $$a+b+2c=even$$ --> $$a+b+even=even$$ --> $$a+b=even-even=even$$ --> a and b are either both odd or both even. Not sufficient.

(2) a + 2b + c is odd --> $$a+2b+c=odd$$ --> $$a+even+c=odd$$ --> $$a+c=odd-even=odd$$ --> a is odd and c is even or vise-versa. Not sufficient.

(1)+(2) If a and b are both odd (satisfies the first statement) and c is even but not a multiple of 4, then abc is NOT divisible by 4 but if a and b are both odd and c is a multiple of 4 (so even), then abc IS divisible by 4. We already have two different answers, thus no need to consider the case when a and b are both even. Not sufficient.

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Hope it helps.

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Re: Working with scenario charts  [#permalink]

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dukenukem wrote:
Hi Everyone,

Not sure if this has been posted before, but here is my question: I'm using the manhattan gmat advanced quant book (just because I tried doing a GMAT practice test and found that I wasn't doing as well as I hoped on the quant - 70th percentile) and I found the suggestion to use scenario charts very interesting. However, when I try using them, I can't get my time to go below 3:30s to 4 mins which I know is too long to be working on any one problem.
I'm sure people have had success with the tool, which is why MGMAT recommends it; however, does anyone have any idea on how to use the chart more efficiently?

As an example:
I'm using the chart for try-it problem 4-7 in the MGMAT advanced quant book:
If a,b and c are integers, is abc divisible by 4?
1. a + b + 2c is even.
2. a + 2b + c is odd.

I have 5 columns: a, b, c, 4th column: to ensure a+b+2c is even and 5th column to check if abc is divisible by 4. I try different values for a, b and c and etc till I find whether abc is always divisible by 4 or not.

MGMAT suggests to look at a, b and c as O/E numbers. I find this would not have been intuitive for me (since the question asks "is it divisible by 4," I would automatically start by trying out numbers.)

Anyways, if anyone has some tips or something that you think I should do different, please feel free to share. Any help will be appreciated!

Thanks!!

Hi,

i you try to put numbers and then find out the answer you will have to make too many sets, which will be very cumbersome and time consuming.

I would directly approach the problem by looking for odd or even numbers.
I would suggest that whenever you find such problems, try to do in terms of odd or even only.
If it is asked that the number is divided by 6, then try for 2 and 3 both.

Hope it helps!!

Regards,
Ankur
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Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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dukenukem wrote:
If a, b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even.
(2) a + 2b + c is odd.

I shall say, be flexible.

statement 1 says: either a & b are both even OR a & b are both odd.
a=3, b=3 c=2 Answer is NO
a=3, b=3, c=4 Answer is YES

Not sufficient

statement 2 says: a+c is odd.
so either of a or c is even
Still insufficient.

On combining both of these info, we are still not sure if any of these is an even multiple of 2.
Not sufficient.
+1 E
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Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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Thank you! And also thanks for those links Bunuel!

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Intern  Joined: 29 Sep 2013
Posts: 5
Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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And apologies, spoiler: You guys are right! E is the right answer. wow so many ways to get there (sad face only cause I didn't realize there were so many methods when solving myself!)

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Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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Can't we solve this problem by knowing that a, b, or c could be zero? Zero is an integer and it is even.
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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Hi FWU,

YES, you COULD use 0 in any of your TESTs and come up with a quick "YES" answer. You still have to be thorough enough with your work to find one of the various "NO" answers that also exists though, so that you can get to the correct answer.

GMAT assassins aren't born, they're made,
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Re: If a, b and c are integers, is abc divisible by 4?  [#permalink]

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dukenukem wrote:
If a, b and c are integers, is abc divisible by 4?

(1) a + b + 2c is even.
(2) a + 2b + c is odd.

$$a,b,c\,\,{\rm{ints}}\,\,\,\,\left( * \right)$$

$${{abc} \over 4}\,\,\mathop = \limits^? \,\,{\mathop{\rm int}}$$

$$\left( {1 + 2} \right)\,\,\,\,$$

$$\left( 1 \right)\,\,\,{{a + b + 2c} \over 2} = {\mathop{\rm int}} \,\,\,\,\,\mathop \Leftrightarrow \limits^{\left( * \right)} \,\,\,\,\,\,{{a + b} \over 2} = {\mathop{\rm int}} \,\,\,\,$$

$$\left( 2 \right)\,\,\,{{a + 2b + c} \over 2} \ne {\mathop{\rm int}} \,\,\,\,\,\mathop \Leftrightarrow \limits^{\left( * \right)} \,\,\,\,\,\,{{a + c} \over 2} \ne {\mathop{\rm int}}$$

$$\left\{ \matrix{ \,{\rm{Take}}\,\,\left( {a,b,c} \right) = \left( {0,0,1} \right)\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\left\langle {{\rm{YES}}} \right\rangle \,\, \hfill \cr \,{\rm{Take}}\,\,\left( {a,b,c} \right) = \left( {1,1,2} \right)\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\left\langle {{\rm{NO}}} \right\rangle \hfill \cr} \right.\,\,\,$$

This solution follows the notations and rationale taught in the GMATH method.

Regards,
Fabio.
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Our high-level "quant" preparation starts here: https://gmath.net Re: If a, b and c are integers, is abc divisible by 4?   [#permalink] 07 Oct 2018, 13:05
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