tricky PS when using numbers

Expert replies

ildude02: Master | Next Rank: 500 Posts; Posts: 320; Joined: Sun Jan 13, 2008 10:00 pm; Thanked: 10 times

by ildude02 » Thu Jul 10, 2008 1:26 pm

One night a certain motel renetd 3/4 of its rooms, including 2/3 of its AC rooms. If 3/5 of its rooms were AC, what % of the rooms that were not renetd were AC?

1. 20
2. 33 1/3
3 35
4 40
5 80

If I tried picking numbers and solving it, I went wrong with it. But I was able to solve it drawing a grid. I would like to see someone solve this with the help of picking numbers. I wanted to see where I went wrong with my numbers approach.

Last edited by ildude02 on Thu Jul 10, 2008 2:23 pm, edited 2 times in total.

Source: Beat The GMAT — Problem Solving | Thu Jul 10, 2008 1:26 pm

lion147: Senior | Next Rank: 100 Posts; Posts: 66; Joined: Sun Jul 06, 2008 9:14 am; Location: Florida; Thanked: 7 times; GMAT Score:710

by lion147 » Thu Jul 10, 2008 1:57 pm

Might there be something wrong with this question?

I ask because I picked a number of rooms - 100 - and got this:

100 rooms total
3/4 * 100 = 75 rented rooms
3/5 * 100 = 60 rooms with a/c
2/5 * 60 = 24 rented rooms with a/c

So, 25 total rooms were not rented, but 36 a/c rooms were not rented either.

Maybe I went wrong somewhere?

ildude02: Master | Next Rank: 500 Posts; Posts: 320; Joined: Sun Jan 13, 2008 10:00 pm; Thanked: 10 times

by ildude02 » Thu Jul 10, 2008 2:04 pm

Sorry, I updated the question . It should be 2/3 of it's air conditioned rooms.

lion147: Senior | Next Rank: 100 Posts; Posts: 66; Joined: Sun Jul 06, 2008 9:14 am; Location: Florida; Thanked: 7 times; GMAT Score:710

by lion147 » Thu Jul 10, 2008 2:13 pm

ildude02 wrote:Sorry, I updated the question . It should be 2/3 of it's air conditioned rooms.

ah, OK, so now the rented rooms with a/c is (2/3)*60 = 40, and 60-40=20 rooms with a/c were not rented, therefore the answer to the question is:

( a/c rooms not rented / total rooms not rented ) * 100 =
(20/25)*100 = 80

ildude02: Master | Next Rank: 500 Posts; Posts: 320; Joined: Sun Jan 13, 2008 10:00 pm; Thanked: 10 times

by ildude02 » Thu Jul 10, 2008 2:22 pm

The mistake I made was, I calculated "2/3 of rented rooms" for AC rented rooms. Somehow, "2/3 of it's airconditioned rooms" got me thinking it's "2/3 of the rented rooms that are AC". That's where I went wrong and came up with 40% instead of 80%.

Thanks for your response.

Featured schoolRound 2 closes Jan 7, 2027 54 days left

Chicago Booth

Since 1982, 10 Booth faculty members have received the Nobel Prize in Economic Sciences, more than any other business school.

#3
U.S. News rank: $175,000
Median base salary: 670
Average GMAT Focus: 635
Students in the class

Apply to Booth Visit chicagobooth.edu See the full profile

Photo: Warren LeMay, CC BY-SA 2.0

GMAT Course Reviews

563 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (563 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review