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A certain one-day seminar consisted of a morning session and [#permalink]
06 Jun 2008, 22:07

1

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A certain one-day seminar consisted of a morning session and an afternoon session. If each of the 128 people attending the seminar attended at least one of the two sessions, how many of the people attended the morning session only?

(1) 3/4 of the people attended both sessions.

(2)7/8 of the people attended the afternoon session.

first I remember using fomula: 128=M+A -(MA), it cannot produce the OA second I use fomula: 128=M+A+MA, after I draw Venn, and it produces the choice the same as OA.

I confused little, can you help me when we use the first? or the first is not correct? Thanks! _________________

Re: Gmatprep, Set, [#permalink]
06 Jun 2008, 22:37

Let's apply formula 128 = m + a - ma 128 = m + 7/8*128 - 3/4*128 m = 128 - 112 + 96 m = 112

Here m is total number of people who attended morning session.To get people who attended only morning session we have to subtract people going in both session from m

So, number of people going only to morning session 112 - 3/4*128 = 16

Re: Gmatprep, Set, [#permalink]
06 Jun 2008, 22:48

sondenso wrote:

A certain one-day seminar consisted of a morning session and an afternoon session. If each of the 128 people attending the seminar attended at least one of the two sessions, how many of the people attended the morning session only?

(1) 3/4 of the people attended both sessions.

(2)7/8 of the people attended the afternoon session.

first I remember using fomula: 128=M+A -(MA), it cannot produce the OA second I use fomula: 128=M+A+MA, after I draw Venn, and it produces the choice the same as OA.

I confused little, can you help me when we use the first? or the first is not correct? Thanks!

OA must be B,

B tells us common + afternoon only 128-7/8*128 should give us required answer

Re: Gmatprep, Set, [#permalink]
09 Jun 2008, 17:44

I think it should be C

(1) 3/4 of the people attended both sessions. Ok, from the 1/4 remaining, how many attended only the afternoon session?

(2)7/8 of the people attended the afternoon session. Not sufficient, cause we dont know how many attendend the morning session too.

Together: Bingo! 7/8-3/4(or 6/8) attended only the afternoon session, so 1/8 attended the afternoon session, 6/8 both sessions, and 1/8 only the morning session.

Re: Gmatprep, Set, [#permalink]
09 Jun 2008, 18:36

ldpedroso wrote:

I think it should be C

(1) 3/4 of the people attended both sessions. Ok, from the 1/4 remaining, how many attended only the afternoon session?

(2)7/8 of the people attended the afternoon session. Not sufficient, cause we dont know how many attendend the morning session too.

Together: Bingo! 7/8-3/4(or 6/8) attended only the afternoon session, so 1/8 attended the afternoon session, 6/8 both sessions, and 1/8 only the morning session.

I saw that is not the OA. Do you have the OE?

What did I do wrong???

I think you should draw Venn, you will see the OA is B, GmatPrep say! _________________