OG 13 79

Expert replies

PGMAT: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Fri Jul 23, 2010 1:30 pm

by PGMAT » Mon Jun 25, 2012 5:48 pm

After driving to a riverfront parking lot, Bob plans to run south along the river, turn around, and return to the parking lot, running north along the same path. After running 3.25 miles south, he decides to run for only 50 minutes more. If Bob runs at a constant rate of 8 minutes per mile, how many miles farther south can he run and still be able to return to the parking lot in 50 minutes?

A. 1.5
B. 2.25
C. 3.0
D. 3.25
E. 4.75

OA: A

Can some one explain in detail how to tackle this problem? Thanks.

Like this post

Source: Beat The GMAT — Problem Solving | Mon Jun 25, 2012 5:48 pm

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Mon Jun 25, 2012 7:46 pm

PGMAT wrote:After driving to a riverfront parking lot, Bob plans to run south along the river, turn around, and return to the parking lot, running north along the same path. After running 3.25 miles south, he decides to run for only 50 minutes more. If Bob runs at a constant rate of 8 minutes per mile, how many miles farther south can he run and still be able to return to the parking lot in 50 minutes?

A. 1.5
B. 2.25
C. 3.0
D. 3.25
E. 4.75

OA: A

Can some one explain in detail how to tackle this problem? Thanks.

8 minutes/1 mile = 50 minutes/x miles
x = 50/8 = 25/4 = 6.25.
Thus, in 50 minutes, Bob can run 6.25 miles.

Since Bob has already run 3.25 miles, the total distance there and back = 3.25 + 6.25 = 9.5 miles.
Distance southward = 9.5/2 = 4.75 miles.
Since Bob has already run 3.25 miles southward, the remaining distance that can be run southward = 4.75-3.25 = 1.5.

The correct answer is A.

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

Like this post

PGMAT: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Fri Jul 23, 2010 1:30 pm

by PGMAT » Mon Jun 25, 2012 7:59 pm

8 minutes/1 mile = 50 minutes/x miles
x = 50/8 = 25/4 = 6.25.
Thus, in 50 minutes, Bob can run 6.25 miles.

Since Bob has already run 3.25 miles, the total distance there and back = 3.25 + 6.25 = 9.5 miles.
Distance southward = 9.5/2 = 4.75 miles.
Since Bob has already run 3.25 miles southward, the remaining distance that can be run southward = 4.75-3.25 = 1.5.

The correct answer is A.

Mitch, great explanation. Thank you.

Like this post

pbrmoney: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Sun Oct 14, 2012 12:38 pm; Thanked: 1 times

by pbrmoney » Sun Oct 14, 2012 12:48 pm

Can someone please address why this problem doesn't require the Distance = Rate x Time formula?

By that, I mean that the first step in the OG explanation is to multiply the Distance (3.25 miles) by the rate (8 mph) which yields the time it took Bob to run the first leg.

I understand how to do the problem according to the explanations above, but I don't understand why or even how we do this problem without employing D=RT. When I see these distance problems, my first inclination is to write out D=RT.

If I did this problem using D=RT, I would end up with 3.25/8, which does not yield 26 minutes. If I tried to approach this problem with D=RT on the test, I'd be in trouble, which is disconcerting.

Like this post

GMAT/MBA Expert

Whitney Garner: GMAT Instructor; Posts: 273; Joined: Tue Sep 21, 2010 5:37 amFollowed by: 74 members; Location: Durham, NC; Thanked: 154 times; GMAT Score:770

by Whitney Garner » Sun Oct 14, 2012 12:57 pm

pbrmoney wrote:Can someone please address why this problem doesn't require the Distance = Rate x Time formula?

By that, I mean that the first step in the OG explanation is to multiply the Distance (3.25 miles) by the rate (8 mph) which yields the time it took Bob to run the first leg.

I understand how to do the problem according to the explanations above, but I don't understand why or even how we do this problem without employing D=RT. When I see these distance problems, my first inclination is to write out D=RT.

If I did this problem using D=RT, I would end up with 3.25/8, which does not yield 26 minutes. If I tried to approach this problem with D=RT on the test, I'd be in trouble, which is disconcerting.

Hi pbrmoney!

This is a very tricky problem because of the UNITS - check out the rate we are given...

If Bob runs at a constant rate of 8 MINUTES per MILE.

As you read problems, ALWAYS pay attention to the units. This should have sent up a red flag - WAIT a minute!! We usually see rates as Distance OVER Time (miles per hour, feet per second, etc.). But this gives us Time OVER Distance. It is the opposite.

So if I want to go from a rate that is Time/Distance - I need to "cancel" that distance in the denominator, so rather than DIVIDE by distance, I would have to Multiply!

So (3.25 miles) x (8 min/mile) = (3.25x8) x (miles x min/miles) = 26 minutes.

Hope this helps clear up that bit of the confusion!

Whit

Whitney Garner
GMAT/GRE/EA Instructor & Anxiety/Accommodations Coach
www.whitneygarner.com

Contributor to Beat The GMAT!

Math is a lot like love - a simple idea that can easily get complicated

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 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