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If Dev works alone he will take 20 more hours to complete a

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If Dev works alone he will take 20 more hours to complete a [#permalink] New post 02 Feb 2010, 05:30
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If Dev works alone he will take 20 more hours to complete a task than if he worked with Tina to complete the task. If Tina works alone, she will take 5 more hours to complete the complete the task, then if she worked with Dev to complete the task? What is the ratio of the time taken by Dev to that taken by Tina if each of them worked alone to complete the task?

A. 4 : 1
B. 2 : 1
C. 10 : 1
D. 3 : 1
E. 1 : 2
[Reveal] Spoiler: OA
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Re: Ratio of Time to complete task [#permalink] New post 02 Feb 2010, 06:21
Yes. Can u pl explain the working?
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Re: Ratio of Time to complete task [#permalink] New post 02 Feb 2010, 08:32
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answer is B:

If Dev works alone he will take 20 more hours to complete a task than if he worked with Tina to complete the task. If Tina works alone, she will take 5 more hours to complete the complete the task, then if she worked with Dev to complete the task? What is the ratio of the time taken by Dev to that taken by Tina if each of them worked alone to complete the task?

Take the time necessary to complete the job by both Dev and Tina to be x.
From the problem statement, time necessary to complete the job by Dev alone is: x+20
From the problem statement, time necessary to complete the job by Tina alone is: x+5

Rate of work when Dev and Tina work together: 1/x
Rate of work when Dev works alone: 1/(20+x)
Rate of work when Tina works alone: 1/(5+x)

The rate of work when Tina and Dev works together is equal to sum of the rates when Tina and Dev work alone:

1/x=1/(20+x)+ 1/(5+x)
When simplified the equation becomes:

X^2=100

X can only be the positive as we talk about tome, so x=10

The necessary ratio is T(Dev)/T(Tina)=(x+20)/(x+5)=30/15=2:1
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Re: Ratio of Time to complete task [#permalink] New post 01 May 2012, 06:46
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Let time taken by Dev to complete the work alone be x days and that by Tina be y days

Work done by Dev in 1 day = 1/x
Work done by Tina in 1 day = 1/y

Work done by Dev & Tina in 1 day = 1/x + 1/y = (x+y)/xy
Thus working together they will complete the work in xy/(x+y) days
Acc. to the ques :

1) "If Dev works alone he will take 20 more hours to complete a task than if he worked with Tina'

x - xy/(x+y) = 20
=> (x^2)/(x+y) = 20.............(i)

2) "If Tina works alone, she will take 5 more hours to complete the complete the task, then if she worked with Dev"

y - xy/(x+y) = 5
=> (y^2)/(x+y) = 5..............(ii)

Dividing (i) by (ii)

x^2/y^2 = 4
=> x/y = 2

Ans : B
Hope it helps
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Re: If Dev works alone he will take 20 more hours to complete a [#permalink] New post 18 Jan 2013, 22:08
jusjmkol740 wrote:
If Dev works alone he will take 20 more hours to complete a task than if he worked with Tina to complete the task. If Tina works alone, she will take 5 more hours to complete the complete the task, then if she worked with Dev to complete the task? What is the ratio of the time taken by Dev to that taken by Tina if each of them worked alone to complete the task?

A. 4 : 1
B. 2 : 1
C. 10 : 1
D. 3 : 1
E. 1 : 2


Solved it in a minute! Here's my solution:

Description of the formula used:
If A and B complete a task in x days, then A alone takes=x+a days and B alone take x+b days
Formula: x=\sqrt{ab}

Plugging in the numbers:
x=\sqrt{20*5}
= 10

=>A=20+10=30 and B=5+10=15
=>a/b=2/1
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Re: If Dev works alone he will take 20 more hours to complete a [#permalink] New post 18 Feb 2014, 05:43
jusjmkol740 wrote:
If Dev works alone he will take 20 more hours to complete a task than if he worked with Tina to complete the task. If Tina works alone, she will take 5 more hours to complete the complete the task, then if she worked with Dev to complete the task? What is the ratio of the time taken by Dev to that taken by Tina if each of them worked alone to complete the task?

A. 4 : 1
B. 2 : 1
C. 10 : 1
D. 3 : 1
E. 1 : 2


Is there any way to solve this problem in a time efficient way?
Experts please advice

Cheers
J
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Re: If Dev works alone he will take 20 more hours to complete a [#permalink] New post 18 Feb 2014, 20:35
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Expert's post
jlgdr wrote:
jusjmkol740 wrote:
If Dev works alone he will take 20 more hours to complete a task than if he worked with Tina to complete the task. If Tina works alone, she will take 5 more hours to complete the complete the task, then if she worked with Dev to complete the task? What is the ratio of the time taken by Dev to that taken by Tina if each of them worked alone to complete the task?

A. 4 : 1
B. 2 : 1
C. 10 : 1
D. 3 : 1
E. 1 : 2


Is there any way to solve this problem in a time efficient way?
Experts please advice

Cheers
J


Let's say when they work together, they take t hrs to complete the work.
Dev takes 20 hrs extra when working alone because Tina did not work for t hrs and that work Dev had to do. Ratio of time take by Dev/Tina to do the same work = 20/t
Tina takes 5 hrs extra when working alone because Dev did not work with her for t hrs and hence his part of work, Tine did in 5 hrs. Ratio of time taken by Dev/Tina to do the same work = t/5
20/t = t/5 (The ratio of time taken by Dev and Tina for the same work will always be the same irrespective of the amount of work)
t = 10
Ratio of time taken by Dev/Tina = 20:10 = 2:1
Answer (B)
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Re: If Dev works alone he will take 20 more hours to complete a [#permalink] New post 18 Mar 2014, 03:58
Again the funda of Rate * Time = Work

Time (Both) = T

For Dev:

Rate (Both) =T

Rate * Time = Work
Rate * T+20 = 1

Rate (Dev) = 1/(T+20)

For Tina:

Rate * Time = Work
Rate * T+5 = 1

Rate (Tina) = 1/(T+5)

Combined Rates = 1/(T+20) + 1/(T+5) = 1/T

Solving for it we will get you will get T=10

We want to know what is Time(D)/Time(T)= (T+20)/(T+5)=2:1
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Re: If Dev works alone he will take 20 more hours to complete a   [#permalink] 18 Mar 2014, 03:58
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