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Machine A takes 10 hours to complete a certain job and starts that job

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Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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Machine A takes 10 hours to complete a certain job and starts that job at 9AM. After one hour of working alone, machine A is joined by machine B and together they complete the job at 5PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?

(A) 15 hours
(B) 18 hours
(C) 20 hours
(D) 24 hours
(E) 35 hours

Kudos for a correct solution.
[Reveal] Spoiler: OA

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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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New post 23 Sep 2015, 22:52
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Let us assume total job = 100 units

A finishes 100 units in 10 hrs(given)

Hence A (working rate)= 10 units /hr

Now Given that A works for 1 hr( so 10 units done) then A and B finish total work in 7 hours. Hence A and B finish 90 units in 7 hours. Of these 7x10=70 units were done by A. Hence B did 20 units in 7 hours.

Hence B(working rate)= 20/7 units /hr

Hence B takes 100x7/20 = 35 hours to complete the job.

Answer E.

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Dom.
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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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A does the job in 10 hrs, so in 1 hr it will complete 1/10 of the total work
When A is joined by B at 10 AM, 9/10 (1 - 1/10)of the total work is left. They both complete this work in 7 hrs.
Therefore, they would have completed the whole work in 70/9 hrs.
From this we can say that:
1/10 + 1/B = 9/70 (where B is the number of hours B takes to complete the job alone)
=> 1/B = 9/70 - 1/10 = 2/70 = 1/35
=> B = 35 hours

Hence the answer is E.
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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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Bunuel wrote:
Machine A takes 10 hours to complete a certain job and starts that job at 9AM. After one hour of working alone, machine A is joined by machine B and together they complete the job at 5PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?

(A) 15 hours
(B) 18 hours
(C) 20 hours
(D) 24 hours
(E) 35 hours

Kudos for a correct solution.


Let x be the time taken by machine B to complete the work alone. Then it's 1 hour work will be 1/x.
Total work done by machine A and B in 1 hour will be
\(\frac{1}{10}+\frac{1}{x}\)
=\(\frac{x+10}{10x}\)

So time taken to complete whole work will be \(\frac{10x}{x+10}\)

Machine A worked for 1 hour before Machine B joined.
Work done by A in 1 hour is 1/10
work left to do is 9/10
Machine A and B working together take 7 hours to complete 9/10 work.
\(\frac{9}{10}*\frac{10x}{x+10}=7\)

or \(9x=7x+70\)
or \(2x=70\)
or \(x=35\)

Answer:- E
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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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A alone can finish work in 10 hours..
Therefore a can do 1/10th of work in 1 hour..

A starts at 9 a.m and works alone for 1 hour. A finishes 1/10th of the work.
Remaining work = 1 - (1/10) = 9/10

A and B together then finish the remaining work (9/10) from 10a.m to 5 p.m = 7 hours
therefore in 1 hour together they finish 9/70 work

Adding Rate of A and B

1/A + 1/B = 9/70
1/10 + 1/B = 9/70
B = 35

Answer E
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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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Bunuel wrote:
Machine A takes 10 hours to complete a certain job and starts that job at 9AM. After one hour of working alone, machine A is joined by machine B and together they complete the job at 5PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?

(A) 15 hours
(B) 18 hours
(C) 20 hours
(D) 24 hours
(E) 35 hours

Another approach:

Working ALONE Machine A takes 10 hours to do the job.
With Machine B's help, the job took 8 hours to complete.
So, Machine B saved Machine A 2 hours of work.

In other words, during the 7 hours that Machine B was helping, Machine B did the work that would have taken Machine A an extra 2 hours to complete.
So, it takes Machine B 7 hours to complete the same amount of work that it takes Machine A to complete in 2 hours.
So, it takes Machine B 7/2 times as long to complete what Machine A can complete

It takes Machine A 10 hours to do the job.
So, the time it takes Machine B to do the job = (10)(7/2) = 35 = E

Cheers,
Brent
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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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Bunuel wrote:
Machine A takes 10 hours to complete a certain job and starts that job at 9AM. After one hour of working alone, machine A is joined by machine B and together they complete the job at 5PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?

(A) 15 hours
(B) 18 hours
(C) 20 hours
(D) 24 hours
(E) 35 hours

Kudos for a correct solution.


VERITAS PREP OFFICIAL SOLUTION:

Using only algebra and the memorized formula for Work problems (\(Work = Rate * Time\) or \(Rate = \frac{Work}{Time}\)), we can break this problem into 3 parts: Machine A alone from 9 AM to 10 AM , Machines A and B from 10 AM to 5 PM, and then the hypothetical Machine B alone. Let’s set up these three steps to see how we can solve this on the scenic path.

Machine A takes 10 hours to do the job, so each hour it works finishes 1/10th of the total job. From 9 AM to 10 AM, Machine A works alone, so at 10 AM, machine B kicks in and 1/10th of the job is done. Ergo, 9/10th of the job is left to complete for both machines.

From 10 AM to 5 PM, 7 hours pass, during which 9/10th of the job gets completed. Thus we can calculate the rate of the machines working together: \(\frac{9}{10} = Rate_{A+B} * 7\) hours. \(Rate_{A+B} = \frac{9}{70}\).

Since we know that \(Rate_A + Rate_B = Rate_{A+B}\) (i.e. rates are additive), we can leverage the fact that we know 2 of these 3 rates to find the third using basic fraction addition. \(\frac{1}{10} + Rate_B = \frac{9}{70}\). Putting them all on a common denominator: \(\frac{7}{70} + Rate_B = \frac{9}{70}\), so \(Rate_B = \frac{2}{70}\), or 1/35.

Now that we have B’s rate of 1/35, we can easily tell that it would take 35 hours to complete the entire job.

The correct answer is (E).

The algebraic solution works fine and gets you the right answer, but there are many moving parts to keep track of and many opportunities for mistakes. Can we get to the same answer but faster using conceptual understanding and avoid the scenic route entirely?

If machine A does 1/10th of the work in an hour, and it works from 9 to 5, then it works for 8 hours and accomplishes 80% of the job on its own. This means that machine B only accomplishes 20% of the job, and it does so in 7 hours (10 AM to 5 PM). If the machine does 1/5 of the job in 7 hours, it will take (7*5) 35 hours to complete 5/5 of the job. Answer choice E, using almost no math whatsoever, but rather by exploiting the logic of the question.

In general, if you see how to solve a problem via algebra and are confident you can solve it in 3 minutes or less, then by all means go for it. However, you can save some time if you really understand how questions are set up and what they are testing. It may not be possible to come up with a handy shortcut on test day because of nerves and stress, but during your preparation take a look at how problems are solved and see if you can find a more elegant solution. All roads lead to Rome, and the more routes you know, the less likely you are to get stuck in unfamiliar territory.
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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New post 23 Feb 2016, 11:39
rate of A=1/10=7/70
rate of A+B=9/70
rate of B=2/70=1/35
B will complete job alone in 35 days
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Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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New post 18 Mar 2016, 21:20
Bunuel wrote:
Machine A takes 10 hours to complete a certain job and starts that job at 9AM. After one hour of working alone, machine A is joined by machine B and together they complete the job at 5PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?

(A) 15 hours
(B) 18 hours
(C) 20 hours
(D) 24 hours
(E) 35 hours

Kudos for a correct solution.

A completes 1/10 part of the work in 1 hour
The remaining work is 9/10; therefore, 7(1/10 + 1/y) = 9/10
Solving y = 35 hours.

Alternatively,
Machine A completes 4/5 of the total work. Remaining 1/5 of the work is completed by Machine B in 7 hours. Therefore, B will take 35 hours to complete the whole work.

Last edited by Mathivanan Palraj on 21 Dec 2017, 05:46, edited 1 time in total.
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Re: Machine A takes 10 hours to complete a certain job and starts that job [#permalink]

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New post 21 Mar 2017, 01:02
Bunuel wrote:
Machine A takes 10 hours to complete a certain job and starts that job at 9AM. After one hour of working alone, machine A is joined by machine B and together they complete the job at 5PM. How long would it have taken machine B to complete the job if it had worked alone for the entire job?

(A) 15 hours
(B) 18 hours
(C) 20 hours
(D) 24 hours
(E) 35 hours

Kudos for a correct solution.


let the total work be x units
rate of work for A = x/10
let the rate for B be y
A worked for 8 hrs and b worked for 7 hrs

therefore
8x/10 +7y=x
7y = 2x/10
y = x/35

thefore time taken by B = x/y = x/x/35 = 35

Option E
Re: Machine A takes 10 hours to complete a certain job and starts that job   [#permalink] 21 Mar 2017, 01:02
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