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ani781: Master | Next Rank: 500 Posts; Posts: 141; Joined: Fri Jun 24, 2011 4:35 am; Location: Edison; Thanked: 12 times; Followed by:1 members

Need help with this DS

by ani781 » Thu Sep 26, 2013 3:42 pm

Hi,
i need help on this question.

During a 10-week summer vacation,was the average(arithmetic mean) number of books that Carolyn read per week greater than the average number of books that Jacob read per week?

1) Twice the average number of books that Carolyn read per week was greater than 5 less than twice the average number of books that Jacob read per week.

2) During the last 5 weeks of the vacation,Carolyn read a total of 3 books more than Jacob.

What is the quickest way to visualize and solve these type of questions ?

Experts , can I get some examples of similar kind , please ... I would like to practice qs on the forum of these types.

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Source: Beat The GMAT — Data Sufficiency | Thu Sep 26, 2013 3:42 pm

mevicks: Master | Next Rank: 500 Posts; Posts: 269; Joined: Thu Sep 19, 2013 12:46 am; Thanked: 94 times; Followed by:7 members

by mevicks » Thu Sep 26, 2013 7:50 pm

https://www.beatthegmat.com/set-24-q-12-t4975.html#34848

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Fri Sep 27, 2013 6:37 am

ani781 wrote:Hi,
i need help on this question.

During a 10-week summer vacation, was the average(arithmetic mean) number of books that Carolyn read per week greater than the average number of books that Jacob read per week?

1) Twice the average number of books that Carolyn read per week was greater than 5 less than twice the average number of books that Jacob read per week.

2) During the last 5 weeks of the vacation,Carolyn read a total of 3 books more than Jacob.

Target question: During a 10-week summer vacation, was the average number of books that Carolyn read per week greater than the average number of books that Jacob read per week?

Let C = average number of books per week that Carolyn read
Let J = average number of books per week that Jacob read

REPHRASED target question: Is C > J ?

Statement 1: Twice the average number of books that Carolyn read per week was greater than 5 less than twice the average number of books that Jacob read per week.
So, statement 1 tells us that: 2C > 2J - 5
There are several pairs of values that satisfy this condition. Here are two:
Case a: C = 2 and J = 3, in which case C < J
Case b: C = 5 and J = 3, in which case C > J
Since we cannot answer the REPHRASED target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: During the last 5 weeks of the vacation, Carolyn read a total of 3 books more than Jacob.
This tells us nothing about the first 5 weeks of the vacation, so we have cannot determine the total number of books read over the 10-week period, which means Carolyn or Jacob could have read the most books.
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
From statement 1, we were able to come up with 2 possible, conflicting cases:
Case a: C = 2 and J = 3, in which case C < J
Case b: C = 5 and J = 3, in which case C > J

In case a, Carolyn's average of 2 books/week, means she read a total of 20 books over the 10-week period, and Jacob's average of 3 books/week, means she read a total of 30 books over the 10-week period. Notice that it's quite possible that, during the last 5 weeks, Carolyn read a total of 3 books more than Jacob. So, case a could satisfy both conditions.
Likewise, case b could satisfy both conditions.

Since the two possible cases, yield different answers to the target question, the combined statements are NOT SUFFICIENT

Answer = E

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pm; Location: Vancouver, BC; Thanked: 5254 times; Followed by:1268 members; GMAT Score:770

by Brent@GMATPrepNow » Fri Sep 27, 2013 6:39 am

ani781 wrote: Experts , can I get some examples of similar kind , please ... I would like to practice qs on the forum of these types.

If you're looking for other DS questions involving statistics, you can find tons of questions by using BTG's tagging feature. For example, here are all of the questions tagged as statistics questions: https://www.beatthegmat.com/forums/tags/ ... statistics

See the left side of that linked page for more tag options.

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com