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A cleaning company mixed 4 cups (quick approach)

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by melguy » Mon Aug 19, 2013 12:30 am
A cleaning company mixed 4 cups of bleach with 7 cups of water to create a cleaning solution on Monday. On Tuesday, the cleaning company mixed 11 cups of bleach with 14 cups of water to create a cleaning solution. By approximately what percent did the ratio of bleach to water in the company's cleaning solution increase from Monday to Tuesday?

9%
12%
38%
50%
72%

Please recommend a quick solution (apart from the change / original * 100).

I am trying to work the problem as 4/7 = .56 and 11/14 ~ .70. But i am getting 25% as answer (which is not even an option).

Thanks for your help!
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by ganeshrkamath » Mon Aug 19, 2013 12:40 am
melguy wrote:A cleaning company mixed 4 cups of bleach with 7 cups of water to create a cleaning solution on Monday. On Tuesday, the cleaning company mixed 11 cups of bleach with 14 cups of water to create a cleaning solution. By approximately what percent did the ratio of bleach to water in the company's cleaning solution increase from Monday to Tuesday?

9%
12%
38%
50%
72%

Please recommend a quick solution (apart from the change / original * 100).

I am trying to work the problem as 4/7 = .56 and 11/14 ~ .70. But i am getting 25% as answer (which is not even an option).

Thanks for your help!
B/W on Monday = 4/7 = 8/14
B/W on Tuesday = 11/14

Percentage increase = (11/14 - 8/14) / (8/14) * 100%
= 3/8 * 100%
= 37.5%

Choose C

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by ganeshrkamath » Mon Aug 19, 2013 12:44 am
melguy wrote:Please recommend a quick solution (apart from the change / original * 100).

I am trying to work the problem as 4/7 = .56 and 11/14 ~ .70. But i am getting 25% as answer (which is not even an option).

Thanks for your help!
One tip:
Try to work with fractions as far as possible. Most problems in GMAT are meant to be solved in fractions.
Converting fractions to decimals wastes valuable time and potentially results in wrong answers.

All the best.
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by melguy » Mon Aug 19, 2013 1:02 am
ganeshrkamath wrote: B/W on Monday = 4/7 = 8/14
B/W on Tuesday = 11/14

Percentage increase = (11/14 - 8/14) / (8/14) * 100%
= 3/8 * 100%
= 37.5%

Cheers
I used this method earlier too but exam pressure does funny things to me...I made a mistake along the way! Thanks for your help!
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by GMATGuruNY » Mon Aug 19, 2013 3:30 am
melguy wrote:A cleaning company mixed 4 cups of bleach with 7 cups of water to create a cleaning solution on Monday. On Tuesday, the cleaning company mixed 11 cups of bleach with 14 cups of water to create a cleaning solution. By approximately what percent did the ratio of bleach to water in the company's cleaning solution increase from Monday to Tuesday?

9%
12%
38%
50%
72%
Note the word in red: APPROXIMATELY.
This means we can BALLPARK.

Monday: bleach/water = 4/7 ≈ 1/2 = 50%.
Tuesday: bleach/water = 11/14 ≈ 10/15 = 2/3 = 66.67%.
Increase = 66.67 - 50 ≈ 17.

Since 30% of 50 = 15, the ratio increases by a bit more than 30%.

The correct answer is C.
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