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Paul, a painter, paints only flowers or cats in his notebook

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by AAPL » Mon Jan 28, 2019 3:46 am

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Answers

A

B

C

D

E

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Difficulty—

Economist GMAT

Paul, a painter, paints only flowers or cats in his notebook. The pictures are drawn in either regular pencil or in charcoal (but not in both.) The total number of pictures in Paul's notebook is 39 greater than the number of cat pictures painted in pencil. How many flower pictures drawn in charcoal are there in the notebook?

1) There are 7 flower pictures drawn in pencil in the notebook.
2) There are 11 cat pictures drawn in charcoal in the notebook.

OA C
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Source: — Data Sufficiency |
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Paul, a painter, paints only flowers or cats in his notebook

by fskilnik@GMATH » Mon Jan 28, 2019 5:08 am
AAPL wrote:Economist GMAT

Paul, a painter, paints only flowers or cats in his notebook. Each picture has flowers or cats (but not both) and is drawn in either regular pencil or in charcoal (but not in both.) The total number of pictures in Paul's notebook is 39 greater than the number of cat pictures painted in pencil. How many flower pictures drawn in charcoal are there in the notebook?

1) There are 7 flower pictures drawn in pencil in the notebook.
2) There are 11 cat pictures drawn in charcoal in the notebook.
Excellent opportunity for the grid (a.k.a table, double matrix, you-name-it)!

Image

$$? = y$$

Following the diagram shown, it is evident that each statement alone has a trivial bifurcation (hence it is insufficient).

$$\left( {1 + 2} \right)\,\,\,\,\left( {7 + x} \right) + \left( {y + 11} \right) = x + 39\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,y\,\,{\text{unique}}\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,{\text{SUFF}}.$$

We follow the notations and rationale taught in the GMATH method.

Regards,
Fabio.
Fabio Skilnik :: GMATH method creator ( Math for the GMAT)
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by Jay@ManhattanReview » Mon Jan 28, 2019 10:20 pm
AAPL wrote:Economist GMAT

Paul, a painter, paints only flowers or cats in his notebook. The pictures are drawn in either regular pencil or in charcoal (but not in both.) The total number of pictures in Paul's notebook is 39 greater than the number of cat pictures painted in pencil. How many flower pictures drawn in charcoal are there in the notebook?

1) There are 7 flower pictures drawn in pencil in the notebook.
2) There are 11 cat pictures drawn in charcoal in the notebook.

OA C
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Thus, (7 + 11) + (x + 11) = x + 39

=> y = 21. Sufficient.

The correct answer: C

Hope this helps!

-Jay
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by Brent@GMATPrepNow » Tue Jan 29, 2019 6:02 am
I thought I'd point out that the approach demonstrated by Fabio and Jay is known as the Double Matrix Method.
This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of paintings, and the two characteristics are:
- flower or cat
- charcoal or pencil

This question type is VERY COMMON on the GMAT, so be sure to master the technique.

To learn more about the Double Matrix Method, watch this video: https://www.gmatprepnow.com/module/gmat- ... ems?id=919

Once you're familiar with this technique, you can attempt these additional practice questions:

Easy Problem Solving questions
- https://www.beatthegmat.com/finance-majo ... 67425.html

Medium Problem Solving questions
- https://www.gmatprepnow.com/module/gmat- ... /video/920
- https://www.beatthegmat.com/posted-speed ... 72374.html
- https://www.beatthegmat.com/motel-t271938.html
- https://www.beatthegmat.com/of-the-appli ... 70255.html
- https://www.beatthegmat.com/opening-nigh ... 64869.html
- https://www.beatthegmat.com/at-least-100 ... 74669.html
- https://www.beatthegmat.com/prblem-solving-t279424.html

Difficult Problem Solving questions
- https://www.gmatprepnow.com/module/gmat- ... /video/946
- https://www.beatthegmat.com/ratio-problem-t268339.html
- https://www.beatthegmat.com/overlapping- ... 65223.html
- https://www.beatthegmat.com/fractions-t264254.html
- https://www.beatthegmat.com/overlapping- ... 64092.html
- https://www.beatthegmat.com/mba/2011/05/ ... question-2

Easy Data Sufficiency questions
- https://www.gmatprepnow.com/module/gmat- ... /video/943
- https://www.beatthegmat.com/for-what-per ... 70596.html
- https://www.beatthegmat.com/ds-quest-t187706.html

Medium Data Sufficiency questions
- https://www.beatthegmat.com/sets-matrix-ds-t271914.html
- https://www.beatthegmat.com/each-of-peop ... 71375.html
- https://www.beatthegmat.com/a-manufacturer-t270331.html
- https://www.beatthegmat.com/in-costume-f ... 69355.html
- https://www.beatthegmat.com/mba/2011/05/ ... question-1

Difficult Data Sufficiency questions
- https://youtu.be/dsCeqF9Kbk8
- https://www.beatthegmat.com/double-set-m ... 71423.html
- https://youtu.be/dOZ9KM1m5Hs
- https://www.beatthegmat.com/sets-t269449.html
- https://www.beatthegmat.com/mba/2011/05/ ... question-3

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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