Work sum

Expert replies

abhirup1711: Senior | Next Rank: 100 Posts; Posts: 66; Joined: Wed Jul 13, 2011 2:27 pmFollowed by: 2 members

by abhirup1711 » Wed Jun 12, 2013 6:32 am

Randy can finish a job in a hrs and Alvin can finish the same in b hrs.If they both work together which of the following represents the portion of the job that Randy will not have to complete?
ab/(a+b)
(a-b)/(a+b)
b/(b-a)
a/(a+b)
b/(a+b)

Like this post

Source: Beat The GMAT — Problem Solving | Wed Jun 12, 2013 6:32 am

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 » Wed Jun 12, 2013 6:59 am

abhirup1711 wrote:Randy can finish a job in a hrs and Alvin can finish the same in b hrs.If they both work together which of the following represents the portion of the job that Randy will not have to complete?
ab/(a+b)
(a-b)/(a+b)
b/(b-a)
a/(a+b)
b/(a+b)

Time and rate are RECIPROCALS.
Since the time ratio for Randy and Alvin = a:b, the rate ratio for Randy and Alvin = b:a.
Implication:
For every b units produced by Randy, a units are produced by Alvin.
Thus, of every a+b units, the fraction produced by Alvin = a/(a+b).

The correct answer is D.

Alternate approach:

Let the job = 10 units.

Let a = 5 hours.
Rate for Randy alone = w/t = 10/5 = 2 units per hour.

Let b = 2 hours.
Rate for Alvin alone = w/t = 10/2 = 5 units per hour.

The combined rate for Randy and Alvin = 2+5 = 7 units per hour.
Of the 7 units produced each hour, 2 will produced by Randy and 5 will be produced by Alvin.
Thus, the fraction of the job produced by Alvin = 5/7. This is our target.
Now we plug a=5 and b=2 into the answers to see which yields our target of 5/7.

A quick scan of the answers reveals that only D works:
a/(a+b) = 5/(5+2) = 5/7.

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

Like this post

sks74: Newbie | Next Rank: 10 Posts; Posts: 8; Joined: Sun Apr 28, 2013 1:02 am

by sks74 » Fri Jun 14, 2013 3:49 am

Hi Mitch,
I am not sure, where I am going wrong.

My approach
----------
Randy can do 1/a job in 1hr.
Alvin can do 1/b job in 1 hr,
If both work together, they can do (1/a + 1/b) job in 1 hr.

Portion of job Randy will not have to do = portion of Job Alvin does = (1/b) / (1/a + 1/b) = a/(a+b)

So answer should be C.

Thanks
-sks74

Like this post

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Fri Jun 14, 2013 5:05 am

sks74 wrote:Hi Mitch,
I am not sure, where I am going wrong.

My approach
----------
Randy can do 1/a job in 1hr.
Alvin can do 1/b job in 1 hr,
If both work together, they can do (1/a + 1/b) job in 1 hr.

Portion of job Randy will not have to do = portion of Job Alvin does = (1/b) / (1/a + 1/b) = a/(a+b)

So answer should be C.

Thanks
-sks74

Your answer of a/(a+b)[/b] looks great.
This matches answer choice D (not C)

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

559 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