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Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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people left Auditorium

by sanju09 » Fri Jan 28, 2011 3:51 am
At 6:00 P.M., there were 305 more people in Auditorium A than there were in Auditorium B. By 8:30 P.M., there were twice as many people in Auditorium B as there had been at 6:00 P.M. Between 8:30 P.M. and 10:00 P.M., 85 people left Auditorium B. If there were 605 people in Auditorium B at 10:00 P.M., how many people were in Auditorium A at 6:00 P.M.?
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by aleph777 » Fri Jan 28, 2011 6:27 am
At 10PM, there are 605 in B, and we know that at 830, there are 2B in B. Between 8:30 and 10, 85 people leave B, So at 10PM 2B-85 = 605. Therefore B = 350. And at 6PM, there are 305 more people in A than in B. Therefore, 350+305 = A = 655.

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by bajwa2307 » Mon Jan 31, 2011 11:35 am
So at 10PM 2B-85 = 605. Therefore B = 350. And at 6PM, there are 305 more people in A than in B. Therefore, 350+305 = A = 655.
2B=605+85

2B=690

B=345

And at 6PM, there are 305 more people in A than in B.

Therefore, 345+305 = A = 650
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by GMATGuruNY » Wed Feb 02, 2011 3:10 am
sanju09 wrote:At 6:00 P.M., there were 305 more people in Auditorium A than there were in Auditorium B. By 8:30 P.M., there were twice as many people in Auditorium B as there had been at 6:00 P.M. Between 8:30 P.M. and 10:00 P.M., 85 people left Auditorium B. If there were 605 people in Auditorium B at 10:00 P.M., how many people were in Auditorium A at 6:00 P.M.?
On the GMAT we could plug in the answer choices, one of which would say that the number of people in A at 6pm was 650.

Answer choice: 650 people in A at 6pm
B at 6pm = 650-305 = 345.
B at 8:30pm = 2*345 = 690.
B at 10pm = 690-85 = 605. Success!
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