GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 21 Nov 2018, 12:58

### 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 November
PrevNext
SuMoTuWeThFrSa
28293031123
45678910
11121314151617
18192021222324
2526272829301
Open Detailed Calendar
• ### All GMAT Club Tests are Free and open on November 22nd in celebration of Thanksgiving Day!

November 22, 2018

November 22, 2018

10:00 PM PST

11:00 PM PST

Mark your calendars - All GMAT Club Tests are free and open November 22nd to celebrate Thanksgiving Day! Access will be available from 0:01 AM to 11:59 PM, Pacific Time (USA)
• ### Key Strategies to Master GMAT SC

November 24, 2018

November 24, 2018

07:00 AM PST

09:00 AM PST

Attend this webinar to learn how to leverage Meaning and Logic to solve the most challenging Sentence Correction Questions.

# When to use the formula for WOrk problems

Author Message
TAGS:

### Hide Tags

Intern
Joined: 14 Feb 2012
Posts: 7
When to use the formula for WOrk problems  [#permalink]

### Show Tags

05 Mar 2012, 22:11
I've observed that the some work problems are solved using the formula AB/A+B(ex 6*7/6+7) while some are just added Example 1/6+1/7,, could any one tell me the difference and appropriate contexts to use the same,, THanks in advance.
Intern
Joined: 14 Feb 2012
Posts: 7
Re: When to use the formula for WOrk problems  [#permalink]

### Show Tags

05 Mar 2012, 22:20
kaka1989 wrote:
I've observed that the some work problems are solved using the formula AB/A+B(ex 6*7/6+7) while some are just added Example 1/6+1/7,, could any one tell me the difference and appropriate contexts to use the same,, THanks in advance.

Ex:
Alex, Bob, and Cathy are painters working for a new housing development project in which all the houses are exactly the same. Cathy can paint an entire house by herself in 8 hours; Bob, in 12 hours; and Alex, in 16 hours. What is the ratio of the time it takes Cathy to paint a house by herself to the time it would take Bob and Alex to paint a house, working together at their usual rate?
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 8580
Location: Pune, India
Re: When to use the formula for WOrk problems  [#permalink]

### Show Tags

08 Mar 2012, 04:53
1
3
kaka1989 wrote:
I've observed that the some work problems are solved using the formula AB/A+B(ex 6*7/6+7) while some are just added Example 1/6+1/7,, could any one tell me the difference and appropriate contexts to use the same,, THanks in advance.

A completes a work alone in 6 hrs and b in 7 hrs. In how many hrs do they complete the work together?
Rate of work of A = 1/6
Rate of work of B = 1/7
You add rates to get their combined rate = 1/6 + 1/7 = (6+7)/6*7

Time taken by them together is the inverse = 6*7/(6+7) (because Work = Rate * Time)
This is the formula you are talking about. We use it in this specific case - where we have individual time taken given and we need to find combined time taken.

When you just add (1/6+1/7), this is the combined rate, not the time taken together.

Alex, Bob, and Cathy are painters working for a new housing development project in which all the houses are exactly the same. Cathy can paint an entire house by herself in 8 hours; Bob, in 12 hours; and Alex, in 16 hours. What is the ratio of the time it takes Cathy to paint a house by herself to the time it would take Bob and Alex to paint a house, working together at their usual rate?
Time it would taken Alex and Bob to paint the house together = 12*16/(12+16) hrs
Time it takes Cathy to paint the house = 8 hrs

Required ratio = 8/12*16/28 = 7:6

Be wary of the formula. You can use it in only this situation. Say, if their rates are given instead of time or total work done is required etc, you cannot use it.
_________________

Karishma
Veritas Prep GMAT Instructor

GMAT self-study has never been more personalized or more fun. Try ORION Free!

Intern
Joined: 14 Feb 2012
Posts: 7
Re: When to use the formula for WOrk problems  [#permalink]

### Show Tags

08 Mar 2012, 21:28
Thanks Karishma ,,, I don't know how i missed that one,
Non-Human User
Joined: 09 Sep 2013
Posts: 8864
Re: When to use the formula for WOrk problems  [#permalink]

### Show Tags

05 Dec 2017, 18:44
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
Re: When to use the formula for WOrk problems &nbs [#permalink] 05 Dec 2017, 18:44
Display posts from previous: Sort by