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 Post subject: GMAT Data Analysis
PostPosted: Thu Jul 26, 2012 11:26 pm 
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A computer retailer sells only TFT and LCD style monitors. According to the graph above, where the x-axis represents the day of the week and the y-axis represents the number of monitors sold, on which day was the ratio of the number of LCD monitors sold to total number of monitors sold the greatest?
A. Monday
B. Tuesday
C. Wednesday
D. Thursday
E. Friday

(C) The number of LCD sales is the difference between total sales and TFT sales. For each day of the week, we can determine the ratio of the number of LCD monitors sold to the total number of monitors sold:
Monday: (18 – 10)/18 = 4/9
Tuesday: (22 – 12)/22 = 5/11
Wednesday: (24 – 12)/24 = 1/2
Thursday: (26 – 16)/26 = 5/13
Friday: (26 – 14)/26 = 6/13.

The greatest fraction is 1/2, so the ratio on Wednesday is the largest.The correct answer is choice (C).
-------------

* let L represent the number of LCD monitors sold
* let T represent the number of TFT monitors sold
Why is the ratio calculated as (L-T)/L? I would expect it to be L/(L+T).

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 Post subject: Re: GMAT Data Analysis
PostPosted: Thu Jul 26, 2012 11:32 pm 
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Posts: 64
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Why is the ratio calculated as (L-T)/L? I would expect it to be L/(L+T).

Please, note, that the blue line is "TOTAL sales" line, not "LCD sales" one.

The ratio is calculated (using your denotation):
[(L + T) -T] / (L + T), where (L + T) is the total sales amount.
Let us simplify:
L / (L + T)

But the number of LCD monitors sold is not directly given in the chart. So we have Total and TFT values. Thus we use the formula
(TotalTFT) / Total

As you can see it is the same as L / (L + T), where L = Total – TFT and Total = L + T.


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 Post subject: Re: GMAT Data Analysis
PostPosted: Thu Jul 26, 2012 11:35 pm 
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How is half the largest share? Its equal for both LCD and TFT.


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 Post subject: Re: GMAT Data Analysis
PostPosted: Thu Jul 26, 2012 11:44 pm 
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Joined: Thu Jul 05, 2012 11:55 am
Posts: 64
questioner wrote:
How is half the largest share? Its equal for both LCD and TFT.
We are NOT looking for the day, on which the LCD share was greater than the TFT share. We are looking for the day, on which the LCD daily share was the greatest among all LCD daily shares throughout the week.

Let's approximate fractions with percentages.
Monday: 4/9 ≈ 44.44%
Tuesday: 5/11 ≈ 45.45%
Wednesday: 1/2 = 50%
Thursday: 5/13 ≈ 38.46%
Friday: 6/13 ≈ 46.15%

As you can see the retailer never sold more LCDs than TFTs, but on Wednesday the LCD share was the greatest among the LCD shares throughout the week (On Thursday it was the lowest).


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