BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

All my Problem Solving Problems in 1 thread

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 49
Joined: Wed Jan 16, 2013 6:06 am
Thanked: 1 times
Followed by:1 members

All my Problem Solving Problems in 1 thread

by DBushkalov » Mon Feb 18, 2013 2:53 pm
Hi guys,

I decided to make a new thread regarding all the problems (from the problem solving part only) that I encounter throughout my preparation. So here is the first one. It concerns task 142 from the Official GMAT Review. The task

On a recent trip, Cindy drove her car 290 miles, rounded to the nearest 10 miles, and used 12 gallons of gasoline, rounded to the nearest gallon. The actual number of miles per gallon that Cindy's car got on this trip must have been between ?

My solution: Since the miles are rounded to the nearest 10 miles ==> the range of the mileage is from 285 to 294 (because 284 rounded to the nearest 10 would yield 280, and 295 would yield 300). The range of the fuel is between 11,5 and 12,4 (because 11,4 yields 11 gallon and 12,5 would yield 13 gallons).

Therefore the actual number of miles per galon must have been between 285/11,5 and 294/12,4.

However the Authors of the OG 13 give the answer: The range is between: 285/12,5 and 295/11,5.

How is this possible, since 12, 5 rounded to the nearest gallon would yield not 12 but rather 13. And 295 rounded to the nearest 10 would actually yield 300 ??
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Mon Feb 18, 2013 3:07 pm
On a recent trip, Cindy drove her car 290 miles, rounded to the nearest 10 miles, and used 12 gallons of gasoline, rounded to the nearest gallon. The actual number of miles per gallon that Cindy's car got on this trip must have been between

A) 290/12.5 to 290/11.5

B) 295/12 to 285/11.5

C) 285/12 to 295/12

D) 285/12.5 to 295/11.5

E) 205/12.5 to 285/11.5
We round UP when we reach the HALFWAY point between two values.
Any value BELOW the halfway point is rounded DOWN.

290 miles, rounded to the nearest 10 miles:
285 ≤ m < 295.
Please note that 295 is the UPPER LIMIT.
Any value below 295 -- even 294.999 -- is rounded DOWN to 290.

12 gallons of gasoline, rounded to the nearest gallon:
11.5 ≤ g < 12.5.
Please note that 12.5 is the UPPER LIMIT.
Any value below 12.5 -- even 12.499 -- is rounded DOWN to 12.

Worst miles per gallon = (lower limit for miles)/(upper limit for gas) = 285/12.5.
Best miles per gallon = (upper limit for miles)/(lower limit for gas) = 295/11.5.
Thus, miles per gallon is between 285/12.5 and 295/11.5.

The correct answer is D.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Senior | Next Rank: 100 Posts
Posts: 49
Joined: Wed Jan 16, 2013 6:06 am
Thanked: 1 times
Followed by:1 members

by DBushkalov » Tue Feb 19, 2013 2:06 pm
After I give my big thank you to Mitch i want to draw the attention to another issue I have with Problem Solving. It concerns Question Nr. 137.

Although I fully understand the way to solve this task and actually managed to solve it myself, I can not answer myself this one question:

How on earth are we expected to:

Read the task and understand it

Make a RTD table and fill it with all the values and the variables

Build a qudratic equation

Solve it

Find the square root of the Descriminant 2704 (yeah you can actually try with a number around 50, still though there are two pretty close calls in the answers: 28 and 24), thus guessing the Square root of the descriminant is risky.

It's absolutely INSANE to expect someone to solve this in 120 seconds, at least from my point of view. Therefore I decided to turn to you guys, since you have way more experience in this field.

Thanx in advance for your feedback people. Have a beautiful and successful day.

D. Bush
Top
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Tue Feb 19, 2013 2:16 pm
At his regular hourly rate, Don had estimated the labor cost of a repair job as $336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned 2$ per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours?

28
24
16
14
12
We can plug in the answers, which represent the estimated time: 28, 24, 16, 14, or 12.
The actual time was 4 hours longer: 32, 28, 20, 18, or 16.
Since 336 is not a multiple of 32, eliminate A.
Since 336 is not a multiple of 20, eliminate C.
336 is not a multiple of 9, since the sum of its digits is not a multiple of 9.
Thus, 336 is not a multiple of 18; eliminate D.

The correct answer must be B or E.

Answer choice B: estimated time = 24, actual time = 28
Estimated payrate = 336/24 = 14.
Actual payrate = 336/28 = 12.
Difference = 14-12 = 2.
Success!

The correct answer is B.

Note that we had to try only ONE answer choice -- a very efficient way to solve the problem.

Moving forward, please post each problem in a separate thread.
Otherwise, readers will assume that the entire thread is dedicated only to the first problem discussed.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Post new topic Post Reply
• Page 1 of 1