When to use the formula for WOrk problems : GMAT Quantitative Section
Check GMAT Club Decision Tracker for the Latest School Decision Releases http://gmatclub.com/AppTrack

 It is currently 17 Jan 2017, 23:32

### 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

# When to use the formula for WOrk problems

Author Message
TAGS:

### Hide Tags

Intern
Joined: 14 Feb 2012
Posts: 7
Followers: 0

Kudos [?]: 0 [0], given: 3

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
Followers: 0

Kudos [?]: 0 [0], given: 3

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: 7119
Location: Pune, India
Followers: 2132

Kudos [?]: 13632 [0], given: 222

Re: When to use the formula for WOrk problems [#permalink]

### Show Tags

08 Mar 2012, 04:53
Expert's post
2
This post was
BOOKMARKED
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
My Blog

Get started with Veritas Prep GMAT On Demand for \$199

Veritas Prep Reviews

Intern
Joined: 14 Feb 2012
Posts: 7
Followers: 0

Kudos [?]: 0 [0], given: 3

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,
GMAT Club Legend
Joined: 09 Sep 2013
Posts: 13435
Followers: 575

Kudos [?]: 163 [0], given: 0

Re: When to use the formula for WOrk problems [#permalink]

### Show Tags

13 Oct 2015, 11:18
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   [#permalink] 13 Oct 2015, 11:18
Similar topics Replies Last post
Similar
Topics:
2 How do you use formula to calculate these combinatorics problems? 4 16 Sep 2014, 04:38
2 Understanding Time & Work formula 5 02 Apr 2014, 02:00
21 Formula Sheet + Practice Problems 8 21 Mar 2011, 12:41
when to use formula n*(First term+last term)/2 to calculate 1 02 Feb 2011, 21:43
16 Two important formulas for mixture problems 7 19 Aug 2009, 05:10
Display posts from previous: Sort by