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pkw209: Master | Next Rank: 500 Posts; Posts: 266; Joined: Mon Oct 19, 2009 9:46 pm; Thanked: 8 times; GMAT Score:690

by pkw209 » Mon Jan 11, 2010 1:38 pm

Hi,

Couldn't figure this one out. Took this from Zuleron's 198 gmat prep questions. Answer is B. Thanks!

127) A seminar consisted of morning session and afternoon session. If each of the 128 people attending attended at least one of the two sessions, how many of the people attended the morning session only?

a. Â¾ attended both sessions
b. 7/8 attended the afternoon session

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Source: Beat The GMAT — Data Sufficiency | Mon Jan 11, 2010 1:38 pm

papgust: Community Manager; Posts: 1537; Joined: Mon Aug 10, 2009 6:10 pmFollowed by: 252 members; Thanked: 653 times

by papgust » Mon Jan 11, 2010 7:48 pm

Total = 128 students.
How many attended morning session?

A. 3/4 attended both sessions.
We cannot determine number of morning and afternoon sessions. Insufficient.

B. 7/8 attended afternoon session
The key here is it doesn't say afternoon session only. It only says afternoon session which includes both sessions.

128 = M + A + Both
128 = M + (7/8 * 128)
128 = M + (112)
M = 16.
Sufficient.

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pkw209: Master | Next Rank: 500 Posts; Posts: 266; Joined: Mon Oct 19, 2009 9:46 pm; Thanked: 8 times; GMAT Score:690

by pkw209 » Wed Jan 13, 2010 11:40 am

awesome.

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Testluv: GMAT Instructor; Posts: 1302; Joined: Mon Oct 19, 2009 2:13 pmFollowed by: 164 members; Location: Toronto; Thanked: 539 times; GMAT Score:800

by Testluv » Wed Jan 13, 2010 7:33 pm

papgust's approach was great--that would also be the approach I would have adopted.

Note that there are two important formulae for counting the total number of objects in sets problems:

Total number of objects = number in at least group 1 + number in at least group 2 - number in both groups + number in neither group.

Notes:

-The reason we subtract both is because we don't want to double count objects. Objects that appear in both group 1 and group 2 have been counted twice--once in count of objects in group 1, and again in the count of objects in group 2.

-The reason we add the objects that are not in either group 1 or group 2 is because those objects have yet to be counted. But note that in many problems we don't have to worry about the "neither" group because there isn't one.

The other formula is:

Total number of objects = number in only group 1 + number in only group 2 + number in both groups + number in neither group

Here we add "both" because we have yet to count the objects appearing in both.

Depending on the information, we want to use the first formula, the second formula or both formulae.

Kaplan Teacher in Toronto

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pkw209: Master | Next Rank: 500 Posts; Posts: 266; Joined: Mon Oct 19, 2009 9:46 pm; Thanked: 8 times; GMAT Score:690

by pkw209 » Tue Jan 19, 2010 1:29 pm

So in instances where there is no "neither" it's ok to disregard and just focus on the three other groups? In other words...

1) Total number of objects = number in at least group 1 + number in at least group 2 - number in both groups

2) Total number of objects = number in only group 1 + number in only group 2 + number in both groups

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Tue Jan 19, 2010 2:16 pm

pkw209 wrote:So in instances where there is no "neither" it's ok to disregard and just focus on the three other groups? In other words...

1) Total number of objects = number in at least group 1 + number in at least group 2 - number in both groups

2) Total number of objects = number in only group 1 + number in only group 2 + number in both groups

Correct - all you've done is set "neither" to 0 and maintained the rest of each formula.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
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tanviet: Legendary Member; Posts: 1404; Joined: Tue May 20, 2008 6:55 pmFollowed by: 2 members; Thanked: 18 times

by tanviet » Sat Jan 23, 2010 1:35 am

THIS IS VEN DIAGRAM. pls, review VEN question in OG

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Warlock007: Senior | Next Rank: 100 Posts; Posts: 41; Joined: Sat Jan 08, 2011 7:34 amFollowed by: 1 member; Thanked: 2 times

by Warlock007 » Tue Aug 16, 2011 9:20 am

papgust wrote:Total = 128 students.
How many attended morning session[you missed "ONLY" here]?

A. 3/4 attended both sessions.
We cannot determine number of morning and afternoon sessions. Insufficient.

B. 7/8 attended afternoon session
The key here is it doesn't say afternoon session only. It only says afternoon session which includes both sessions.

128 = M + A + Both
128 = M + (7/8 * 128)
128 = M + (112)
M = 16.
Sufficient.

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