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S98-05

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S98-05 [#permalink]

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New post 16 Sep 2014, 01:52
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  55% (hard)

Question Stats:

56% (01:26) correct 44% (01:57) wrong based on 32 sessions

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A survey was sent to 80 customers, 7 of whom responded. Then the survey was redesigned and sent to another 63 customers, 9 of whom responded. By approximately what percent did the response rate increase from the original survey to the redesigned survey?

A. 2%
B. 5%
C. 14%
D. 28%
E. 63%

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Re S98-05 [#permalink]

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New post 16 Sep 2014, 01:52
Official Solution:

A survey was sent to 80 customers, 7 of whom responded. Then the survey was redesigned and sent to another 63 customers, 9 of whom responded. By approximately what percent did the response rate increase from the original survey to the redesigned survey?

A. 2%
B. 5%
C. 14%
D. 28%
E. 63%


To calculate the percent increase of any value, we use this formula: \(\frac{\text{New} - \text{Old}}{\text{Old}}\). In other words, we subtract the original value \(\text{Old}\) from the increased value \(\text{New}\), then divide the result by the original value \(\text{Old}\). Even though the values themselves may be written using percents, we must be sure to divide by the original value.

In this problem, the original response rate is \(\frac{7}{80}\). The new response rate is \(\frac{9}{63}\), or \(\frac{1}{7}\). The difference is \(\frac{1}{7} - \frac{7}{80}\). We convert to common denominators: \(\frac{80}{560} - \frac{49}{560} = \frac{31}{560}\). Now, we divide by the original value, \(\frac{7}{80}\). Doing so is the same as multiplying by \(\frac{80}{7}\). This gives us \(\frac{31}{560}*\frac{80}{7} = \frac{31}{49}\). Estimating, we can see that this fraction is slightly more than \(\frac{30}{50}\), which is \(\frac{60}{100}\) or 60%. Thus, the correct answer must be (E): 63%.

Note that the response rates may be written as percents. For instance, the original response rate \(= \frac{7}{80} = 0.0875 = 8.75%\). Likewise, the new response rate \(= \frac{9}{63} = \frac{1}{7} \approx 14.3%\). However, if you write these rates this way, do not simply take the difference! That difference, approximately 5 or 6%, does NOT represent the percent increase in the response rate. You must divide by the original response rate.


Answer: E
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Re: S98-05 [#permalink]

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New post 17 Jan 2018, 05:46
Another faster method could be making the basis of comparison same. If 7 out of 80 have responded, how many have responded out of 100 based on 7/80 rate. This would be about (7*100)/80 = 35/4; Similarly, (9*100)/63 = 1/7. Therefore ( (1/7) - (35/4) )/ 1/7, which is about (14-8)/8, which is roughly 65%. Closest answer is 63 as we dont need to consider decimals for quick calculation.
Re: S98-05   [#permalink] 17 Jan 2018, 05:46
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