It is currently 22 Jun 2017, 18:49

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

Q: If Sam can finish a job in 3 hours and Mark can finish a

Author Message
Manager
Joined: 05 May 2005
Posts: 78
Q: If Sam can finish a job in 3 hours and Mark can finish a [#permalink]

Show Tags

12 May 2005, 17:56
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Q:
If Sam can finish a job in 3 hours and Mark can finish a job in 12 hours, in how many hours could they finish the job if they worked on it together at their respective rates?

(A) 1
(B) 2 2/5
(C) 2 5/8
(D) 3 1/4
(E) 4

What may be different ways to solve this?
Senior Manager
Joined: 15 Mar 2005
Posts: 418
Location: Phoenix

Show Tags

12 May 2005, 18:06
One way:
Combined rate = Rate of 1 + Rate of 2
= 1/3 + 1/12 = 5/12
Therefore total time taken = 12/5 = 2, 2/5 hours

Another way:
Mark does the job in 12 hours. Now he adds someone who can do the job in 12 hours, ie 4 times faster than himself. Thus, the rate of job being done should become 5 times faster than before, and time should decrease 5 times. Therefore, total time = 12/5 = 2, 2/5

Another way:
San does the job in 3 hours. Now he adds someone who can do it at 1/4th the rate. Thus his rate increases from 1 to 1.25 and time should therefore decrease 1.25 times. Therefore total time = 3/1.25 = 2.4 = 2, 2/5

Another way:
In one hour, Mark does 1/12th work, and Sam does 1/3rd = 5/12th work.
Therefore number of hours required = 12/5 = 2, 2/5

Hope that helps.
_________________

Who says elephants can't dance?

Manager
Joined: 05 May 2005
Posts: 78

Show Tags

12 May 2005, 19:06
Yes, this definitely helps. In the last method, why is 5/12 flipped to 12/5? Thanks.
Senior Manager
Joined: 15 Mar 2005
Posts: 418
Location: Phoenix

Show Tags

12 May 2005, 20:03
above720 wrote:
Yes, this definitely helps. In the last method, why is 5/12 flipped to 12/5? Thanks.

The "flip" accounts for hours and "proportion of work done".

Simply put, if you do 1/2 of the work every hour, you take 2 hours to do it all. Or if you do 1/3rd of the work every hour, you take 3 hours to finish it all.

So since Mark and the Sam do 5/12th of the work every hour, they take 12/5 hours to finish it all.

Hope that helps.
_________________

Who says elephants can't dance?

Manager
Joined: 05 May 2005
Posts: 78

Show Tags

13 May 2005, 23:43
THANK YOU, this really helps me understand this concept.
Manager
Joined: 07 Mar 2005
Posts: 93

Show Tags

14 May 2005, 16:34
the way I always do these problems is by picking some arbitrary number that both rates can divide into.

For example, suppose the job = 12 units.

So every hour Sam completes 4 units of the job.

Every hour Mark completes 1 unit of the job.

Combine the rates = 5 units an hour.

12/5 = 2.4 or 2 and 2/5
Manager
Joined: 04 Mar 2005
Posts: 106
Location: NYC

Show Tags

19 May 2005, 08:00
above720 wrote:
Q:
If Sam can finish a job in 3 hours and Mark can finish a job in 12 hours, in how many hours could they finish the job if they worked on it together at their respective rates?

(A) 1
(B) 2 2/5
(C) 2 5/8
(D) 3 1/4
(E) 4

What may be different ways to solve this?

B

Time taken to complete the work together = 12*/15 = 2 2/5
SVP
Joined: 03 Jan 2005
Posts: 2233

Show Tags

20 May 2005, 00:03
The easiest way, if you ask me, is to say, in 12 hours Sam can get 4 jobs down and Mark can get 1 job down. So they get 5 jobs down in 12 hours. In other words they get 1 job done in 12/5 hours. Very straight forward.
_________________

Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.

20 May 2005, 00:03
Display posts from previous: Sort by