40 employees take 30 days,working at 8 hrs per day,to complete a task.

Question

40 employees take 30 days,working at 8 hrs per day,to complete a task. 40 employees start the work but after 10 days,20 leave and are replaced by employees who are 1/2 as productive.How many hours per day should the new team work if the work has to be completed in the scheduled timeline?

1. 12
2. 20
3. 10.6
4. 6
5. 30

Source: Experts' Global

1. 12
2. 20
3. 10.6
4. 6
5. 30

Source: Experts' Global

KarishmaB · Accepted Answer

Using Ratios:

When half the employees are exchanged for 1/2 as productive employees, the total rate of work becomes 3/4 of original.

Time taken = 4/3 of original for same work

Original time taken for remaining work would have been = 20*8 = 160 hrs
New time taken for remaining work = 160 * 4/3 hrs = 640/3 hrs
In 20 days, this implies 640/3*20 = 10.6 hrs per day

Answer (C)

Rizz · Answer

Total man hour required to complete the task = 40*30*8 = 9,600 man hours

Situation 1:  
40 employees works 8 hour each for 10 days = 40*10*8 = 3,200 man hours completed

Situation 2:  
20 employees (Set 1) replaced by another set of 20 employees (Set 2) who are half productive as Set 1

Here, the new group has been asked to complete the task within 30 days by working additional hours in a day. Remaining no. of days is  20 .

Hence, the new group has to complete 6,400 man hours in 20 days. 20 employees in set 1 is twice productive as 20 employees in set 2. All employees will work for same no. of hours in the day.

20 employees in set 1 * 20 days *  x  days + 1/2 (20 employees in set 2 * 20 days *  x  days) = 6,400
400  x  + 200  x   = 6,400
600  x  = 6,400
 x  = 10.66

So the answer is C

EMPOWERgmatRichC · Answer

Hi All,

We're told that 40 employees take 30 days (working at 8 hrs per day) to complete a task. 40 employees start the work - BUT after 10 days, 20 workers leave and are replaced by employees who are 1/2 as productive. We're asked for the number hours per day that the new team must work to complete the job in the scheduled timeline. With these types of rate questions, it helps to first figure out the total amount of 'work' needed to complete the job, then use that information with whatever other information you've been given.

The original team would need (40)(30)(8) = 9600 worker-hours to complete the job

For the first 10 days, all 40 employees work as planned, so (40)(10)(8) = 3200 worker-hours are completed, leaving 9600 - 3200 = 6400 worker-hours to go

20 of the 40 employees are replaced with workers who are HALF as productive, meaning that each of those employees completes 1/2 a worker-hour of work per 1 hour. With those 20 replacement workers, the 'new' team of 40 employees will complete LESS work per hour than the original team did.

New team: 
20 original workers complete 20 worker-hours per hour 
20 new workers complete 10 worker-hours per hour 
20 + 10 = 30 worker-hours completed per hour

With 20 days remaining and 6400 worker-hours to go... 
6400/20 = 320 worker-hours must be completed each day.

At the new rate, that would require... 
320/30 = 10 2/3 hours of work per worker each day

Final Answer:  C

GMAT assassins aren't born, they're made, 
Rich

chetan2u · Answer

Hi...

the TIMER shows all have gone wrong, so here is something which may help ..

two ways..
 1) LOGICAL... 
for the remaining days 20 are working for 8 hrs and other 20 will require to work for 16hrs..
 so all 40 work for entire 8 hrs and then share the remaining work of 16-8=8 hrs.. 
these 8 hrs are of  the speed of 1/2 productive  and if we add full productive person, it is SAME  as adding 2 half-productive persons..

so these 8 hrs is being shared by 1+2=3 person, MEANING each person requires to do  \frac{8}{3}=2.66  hrs
so the hours required is  8+2.66=10.66 
C

PROPER method..  
40 require  30*8=240  hrs, so 1 full productive will require  240*40 h.....1hr work =  \frac{1}{240*40} , so 
1 hr work of half productive =  \frac{1}{2}*\frac{1}{240*40}= \frac{1}{480*40} ..
 combined work of these two - set of productive+half productive  =  \frac{1}{240*40}+\frac{1}{480*40} = \frac{3}{480*40} 
now there are 20 sets so 1 hr work of all 40 =  20*\frac{3}{480*40}=\frac{1}{320} 
 so they require to work for 320 h for complete work  but remaining work is  \frac{30-10}{30}=\frac{2}{3} , this will be completed in  \frac{320*2}{3} 
and these hours are equally spread across 20 days, so per day =  \frac{320*2}{3*20}=\frac{32}{3} = 10.66

C

JeffTargetTestPrep · Answer

The rate for the 40 workers is 1/(30 x 8) = 1/240 task/hour

So after 10 days, the amount of work completed is 1/240 x 10 x 8 = 80/240 = 1/3 of the job and thus 2/3 is left to be completed.

Since 20 workers leave, the rate of the remaining 20 workers is 1/2 x 1/240 = 1/480 task/hour and the 20 new workers who join in have a rate that is half of 1/480, or 1/960, task/hour. Thus the new rate of the 40 workers (20 original and 20 new workers) is 1/480 + 1/960 = 3/960 = 1/320 task/hour.

They still have to finish the task in 20 more days. If we let n = the number of hours they work per day, then it must be true that:

1/320 x 20 x n = 2/3

1/16 x n = 2/3

n = 2/3 x 16

n = 32/3 = 10 ⅔ ≈ 10.6

Alternate Solution:

The number of worker-hours required for the entire job is 40 workers x 30 days x 8 hours/day = 9600 worker-hours. In the first 10 days, the workers have accomplished 40 workers x 10 days x 8 hours/day = 3200 worker-hours, leaving 6400 worker-hours to be accomplished.

The remaining work will be accomplished by 20 original workers plus 20 new workers who work at half-speed. The total amount of work accomplished, then, is equivalent to the work of 30 original workers.

With 6400 worker-hours needing to be done by (the equivalent of) 30 workers, we see that each worker will have to work for 6400/30 ≈ 213.33 hours. This needs to be done in 20 days, so each worker will have to work 213.33/20 ≈ 10.6 hours per day.

Answer: C

chetan2u · Answer

Hi...

the TIMER shows all have gone wrong, so here is something which may help ..

two ways..
 1) LOGICAL... 
for the remaining days 20 are working for 8 hrs and other 20 will require to work for 16hrs..
 so all 40 work for entire 8 hrs and then share the remaining work of 16-8=8 hrs.. 
these 8 hrs are of  the speed of 1/2 productive  and if we add full productive person, it is SAME  as adding 2 half-productive persons..

so these 8 hrs is being shared by 1+2=3 person, MEANING each person requires to do  \frac{8}{3}=2.66  hrs
so the hours required is  8+2.66=10.66 
C

Buttercup3 · Answer

generis  please help with this question using your method.

generis · Answer

Buttercup3 - Sure. The approach, though, works when all workers are equally productive. When all the workers are not equally productive, we have to finesse the formula a bit.* These employees are all going to be men. Time is in hours. Rewrite the standard work formula RT=W by adding (# of workers) to LHS: (# of workers) * (Rate) * (Time) = Work Scenario 1 , FIRST 10 DAYS - equally productive workers 1) Find the rate of each man # * R * T = W # = 40 R = ?? T = 240 (30 days * 8 hrs per day) W = 1 40 * R * 240 = 1 R = \frac{1}{(40)(240)}=\frac{1}{9600} = rate of individual man 2) How much work is finished?** W = # * R * T # = 40 R = \frac{1}{9600} T = 80 (10 days * 8 hrs per day) W = 40 * \frac{1}{9600} * 80 W = \frac{3200}{9600}=\frac{1}{3} Work is \frac{1}{3} finished, \frac{2}{3}W remains Scenario 2 : NEXT 20 DAYS, workers not equally productive 20 men leave. Replaced by 20 men who are half as productive Adding individual rates (fast and slow) and simply multiplying by number of men (40) will not work because the workers are not equally productive. chetan2u shows one way to handle the different levels of productivity. Pair one fast man with one slow man, add their rates, then multiply by the number of pairs (20 pairs). You can also find each group's rate (or each "set's" rate), then add them. Group rate in this context is simply # (of men) * R (individual man) -- i.e. two of the three variables on LHS Find each group's / set's rate, then add • FAST set's rate: ( # * R) = (20 * \frac{1}{9600}) = \frac{20}{9600} • SLOW set's rate: ( # * R * \frac{1}{2}) = (20*\frac{1}{9600}*\frac{1}{2})= \frac{20}{(9600)(2)} = \frac{20}{19200} FAST set + SLOW set = new rate = R_2 R_{2}= (\frac{20}{9600}+\frac{20}{19200})=(\frac{40}{19200} + \frac{20}{19200})= \frac{60}{19200}=\frac{6}{1920}= \frac{1}{320} = R_2 With R_2 , we have calculated ( # * R) on LHS. We can substitute R_{2} for those two variables, thus R_2 * T = W Number of hours per day ? Total hours divided by number of days Total hours , R_2 * T = W , so T = \frac{W}{R_{2}} W = \frac{2}{3} R_{2} = \frac{1}{320} T = \frac{\frac{2}{3}}{\frac{1}{320}}= \frac{2}{3} * 320 = \frac{640}{3} total hours Hours per day : \frac{TotalHours}{NumberOfDays} \frac{\frac{640}{3}}{20}= \frac{640}{60} = \frac{64}{6} = \frac{32}{3} \approx{10.66}\approx{10.6} hours per day Answer C Hope that helps.

* If all 40 workers were replaced by workers half as productive, you would stay with an unmodified (# * R * T) = W
You would: 1) Find "normal" rate 2) Find work finished after 10 days; 3) Find the slower rate (1/2 * R); 4) use the formula straightforwardly. No modifications
**We know this already, from direct proportionality between time and work when rate is constant.

mbafinhopeful · Answer

Total work  = 40 * 30 * 8 = 9600  units

Work done by  40  employees in  10  days  = 40*10*8 = 3200  units.

Remaining work  = 9600-3200 = 6400  units.

In order to complete the work on scheduled time  40  employees must work for  \frac{6400}{(40*20)} = 8  hours a day, but since  20  of the current employees are only  \frac{1}{2}  as efficient,  20  employees with  100\%  efficiency would equal to  10  with  50\%  efficiency, hence the total number of employees can be treated as  30 .

So, the no. of hours to be worked =  \frac{6400}{(30*20)} = \frac{6400}{600} = 10.6 . Ans - C.

hoang221 · Answer

Here is my way of solving this problem:

In the first 10 days, the original set of employees did 1/3 of the work, so the remain is 2/3 of the work. After 10 days, 20 of the employees will be replaced with new employees that is half as productive. So:

Original: 40 employees --- 8 h/day --- 30 days --- 1 work
Now : 40 employees --- 8 h/day --- 30 days --- 1/2 + 1/4 = 3/4 work (because new employees are half as productive)

We know from the problem that the new crew needed to increase hours per day to complete the remaining work in 20 days, hence:

40 employees --- 8 h/day --- 30 days --- 3/4 work (the work rate of new crew)
40 employees --- x h/day --- 20 days --- 2/3 work (the remaining work needed to be done in 20 days)

We got the following equation: x/8 * 20/30 = (2/3) / (3/4), equal to x = (12 * 8)/9 = 10.66

Answers C
There was a link to an article to easily solve this kind of problem by Karishma but i can't post in since my post count is way too low

generis · Answer

hoang221  , welcome!

Please send me a PM with the link. I will be happy to post it for you here, with credit to you and  VeritasPrepKarishma .

generis · Answer

The excellent article to which  hoang221  refers is  Work-Rate Questions Made Easy!

RaghavDhamija · Answer

I tried it by unitary method :

if total work expected per day is 16(8+8)

but because of 1/2 productivity from 20 people

12(8+4) units of work is done in 8 hours
1 unit in 8/12 hours
16 units in (8/12)*16 = 10.66

GMATGuruNY · Answer

Let the rate for each of the 40 employees = 2 widgets per hour, implying the following:
Hourly rate for 40 employees = 2*40 = 80 widgets
Work produced over each 8-hour day = 8*80 = 640 widgets
Total amount of work to be produced for the 30-day job = 30*640 = 19200 widgets

Since the daily rate for 40 workers = 640 widgets, the work produced over the first 10 days = 10*640 = 6400 widgets.
Remaining work = 19200 - 6400 = 12800 widgets

Since the 20 replacement workers are half as productive as the 20 original employees, the resulting rate for the new team = 20*1 + 20*2 = 60 widgets per hour.
To complete 12800 widgets in the remaining 20 days, the daily work required  = \frac{12800}{20} = 640  widgets.
For 640 widgets to be produced each day at a rate of 60 widgets per hour, the number of hours required  = \frac{640}{60}  ≈ 10.6
 C

KarishmaB · Answer

Yes, this method has its roots in variation. The link is shared by generis above and here is the link to the posts: https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2015/1 ... made-easy/ https://www.gmatclub.com/forum/veritas-prep-resource-links-no-longer-available-399979.html#/2013/0 ... variation/ You can break down the information given to make two whole scenarios. 40 men......30 days ...... 8 hrs ...... 1 work 30 men ..... 20 days ....... ?? hrs..... 2/3 work (Note that after 10 days, 1/3rd the work will be done so 2/3rd work will be left. Also, if 20 men leave and are replaced by 20 men half as productive, it is like getting back only 10 men) No of hrs = 8 * (40/30) * (30/20) * (2/3) = 10.66 hrs

PSKhore · Answer

Total Work = 40*30*8 = 9600 man hours
Work done in the first 10 days = 40*10*8 = 3200
Remaining work = 6400 man hours

We have 20 effective employees and 20 that are half as productive.
Those that are half as productive (20 employees) will do the work equivalent to 10 effective employees

Therefore, to do the remaining work, we have 30 effective employees. As a result, the number of hours per day -

6400 = 30*20*x
x = 10.67 hours

40 employees take 30 days,working at 8 hrs per day,to complete a task.

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40 employees take 30 days,working at 8 hrs per day,to complete a task.

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