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Manager  Joined: 02 Dec 2012
Posts: 174
It would take one machine 4 hours to complete a large production order  [#permalink]

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7
10 00:00

Difficulty:   5% (low)

Question Stats: 81% (00:56) correct 19% (00:59) wrong based on 1138 sessions

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It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) $$\frac{7}{12}$$

(B) $$1 \frac{1}{2}$$

(C) $$1 \frac{5}{7}$$

(D) $$3 \frac{1}{2}$$

(E) 7
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Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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5
1
Walkabout wrote:
It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) 7/12
(B) 1 1/2
(C) 1 5/7
(D) 3 1/2
(E) 7

We can classify this problem as a “combined worker” problem. To solve this type of problem we should use the formula:

Work (done by worker 1) + Work (done by worker 2) = Total Work Completed

It takes machine one 4 hours to complete a job, so the rate of machine one is ¼. It takes machine two 3 hours to complete a job, so the rate of machine two is 1/3. Since we know they are both working together to complete the job, we can label this unknown time as “t” for each machine during the time that both machines are working together. Since rate x time = work, we can multiply to get the work completed for each machine. Finally, we can plug our two work values into the combined work formula and determine t. Since the job is completed, the total work completed is 1.

Work (done by worker 1) + Work (done by worker 2) = Total Work Completed

(1/4)t + (1/3)t = 1

Multiplying the entire equation by 12 gives us:

3t + 4t = 12

7t = 12

t = 12/7 = 1 5/7

Answer is C.
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##### General Discussion
Math Expert V
Joined: 02 Sep 2009
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Walkabout wrote:
It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) $$\frac{7}{12}$$

(B) $$1 \frac{1}{2}$$

(C) $$1 \frac{5}{7}$$

(D) $$3 \frac{1}{2}$$

(E) 7

The rate of the first machine is 1/4 job per hour;
The rate of the second machine is 1/3 job per hour;

Thus, the combined rate of the machines is $$\frac{1}{4}+\frac{1}{3}=\frac{7}{12}$$ job per hour, which means that it takes $$\frac{1}{(\frac{7}{12})}=\frac{12}{7}=1 \frac{5}{7}$$ hours both machines to do the job.

Answer: C.
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Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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Bunuel wrote:
Walkabout wrote:
It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) 7/12
(B) 1 1/2
(C) 1 5/7
(D) 3 1/2
(E) 7

The rate of the first machine is 1/4 job per hour;
The rate of the second machine is 1/3 job per hour;

Thus, the combined rate of the machines is 1/4+1/3=7/12 job per hour, which means that it takes 1/(7/12)=12/7 hours both machines to do the job.

Answer: C.

Suppose we change the wording of the problem to "It would take 2 machines 4 hours together to complete a large production" How would you solve for that ? I'm trying to better understand the nature of the problem. Thanks. My logic says the rate is still the same since it still is 4 hours. If so than what does it change?
Intern  B
Joined: 19 Jan 2012
Posts: 1
Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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how can the answer be C
in options dere is no option of 12/7
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Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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arpana10july wrote:
how can the answer be C
in options dere is no option of 12/7

there is a typo in choice.

machine A takes 4 hr
machine B takes 3 hr
then together they will take = product /sum =$$(4*3)/(4+3)$$
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Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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arpana10july wrote:
how can the answer be C
in options dere is no option of 12/7

12/7 is equivalent to 1 and 5/7
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Joined: 12 Sep 2015
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Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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Top Contributor
Walkabout wrote:
It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) 7/12
(B) 1 1/2
(C) 1 5/7
(D) 3 1/2
(E) 7

Another approach is to assign the ENTIRE job a certain number of units.
The least common multiple of 4 and 3 is 12.
So, let's say the ENTIRE production order consists of 12 widgets.

It would take one machine 4 hours to complete a large production...
Rate = output/time
So, this machine's rate = 12/4 = 3 widgets per hour

...and another machine 3 hours to complete the same order.
Rate = units/time
So, this machine's rate = 12/3 = 4 widgets per hour

So, their COMBINED rate = 3 + 4 = 7 widgets per hour.

Working simultaneously at their respective constant rates, to complete the order?
Time = output/rate
= 12/7 hours

Answer:

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It would take one machine 4 hours to complete a large production order  [#permalink]

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JeffTargetTestPrep Bunuel When we are given t as 3 and 4 hours why are we still assigning an unknown variable?
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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Hi boomgoesthegmat,

In "work" questions, there are 2 common ways to get to the correct answer (although there are several different ways to "do the math"):

1) When there are 2 entities (people, machines, etc.) working on a task together, use the Work Formula.
2) Convert the individual rates of the 2 (or more) entities, combine and be sure to answer the question that's asked.

In this question, here's how you could use the two options mentioned:

We're told...
Machine A = 4 hours to complete an order
Machine B = 3 hours to complete the same order

We're asked how long it would take the two machines, WORKING TOGETHER, to complete the order.

1) Using the Work Formula: (A)(B)/(A+B).....

(4)(3)/(4+3) = 12/7 hours to complete the job

2) Using the individual rates:

Machine A:
4 hours to do the entire job --> 1 hour to do 1/4 of the job
Machine B:
3 hours to do the entire job --> 1 hour to do 1/3 of the job

In 1 hour, 1/4 + 1/3 = 7/12 of the job is done

**Note: this calculation tells you the FRACTION of the JOB that is complete in 1 HOUR**

Since there is 1 job to complete.....1/(7/12) = 12/7 hours to complete the job

Either way, the Final Answer is...

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Senior PS Moderator V
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It would take one machine 4 hours to complete a large production order  [#permalink]

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adkikani wrote:
JeffTargetTestPrep Bunuel When we are given t as 3 and 4 hours why are we still assigning an unknown variable?

Hi adkikani

The reason we are assigning t(an unknown variable) for time, because we don't
know how much time will be take to complete the entire work, when working together.

When we are given 3 and 4 hours as time, it means that the individual machines
will complete the entire work(working alone) in 3 and 4 hours respectively.

Alternate approach

Since it takes one machine 4 hours to complete the order and the second machine takes
3 hours to complete the order, we can assume the work done to be 60 units.

The individual rate of the first machine to complete the work will be 15 units/hour
and the rate of the second machine is 20 units/hour. So, together they will complete 35 units in an hour.

Therefore, the time taken for both the machines(combined) is $$\frac{60}{35} = \frac{12}{7} =1\frac{5}{7}$$(Option C)

Hope that helps!
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Re: It would take one machine 4 hours to complete a large production order  [#permalink]

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Bunuel wrote:
Walkabout wrote:
It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) $$\frac{7}{12}$$

(B) $$1 \frac{1}{2}$$

(C) $$1 \frac{5}{7}$$

(D) $$3 \frac{1}{2}$$

(E) 7

The rate of the first machine is 1/4 job per hour;
The rate of the second machine is 1/3 job per hour;

Thus, the combined rate of the machines is $$\frac{1}{4}+\frac{1}{3}=\frac{7}{12}$$ job per hour, which means that it takes $$\frac{1}{(\frac{7}{12})}=\frac{12}{7}=1 \frac{5}{7}$$ hours both machines to do the job.

Answer: C.

Hi Bunuel can you help to understand the logic behind this $$\frac{1}{(\frac{7}{12})}=\frac{12}{7}=1 why do you flip \frac{7}{12}$$ in order to know how much time will it take both to complete Senior PS Moderator V
Joined: 26 Feb 2016
Posts: 3386
Location: India
GPA: 3.12
It would take one machine 4 hours to complete a large production order  [#permalink]

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dave13 wrote:
Bunuel wrote:
Walkabout wrote:
It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

(A) $$\frac{7}{12}$$

(B) $$1 \frac{1}{2}$$

(C) $$1 \frac{5}{7}$$

(D) $$3 \frac{1}{2}$$

(E) 7

The rate of the first machine is 1/4 job per hour;
The rate of the second machine is 1/3 job per hour;

Thus, the combined rate of the machines is $$\frac{1}{4}+\frac{1}{3}=\frac{7}{12}$$ job per hour, which means that it takes $$\frac{1}{(\frac{7}{12})}=\frac{12}{7}=1 \frac{5}{7}$$ hours both machines to do the job.

Answer: C.

Hi Bunuel can you help to understand the logic behind this $$\frac{1}{(\frac{7}{12})}=\frac{12}{7}=1 why do you flip \frac{7}{12}$$ in order to know how much time will it take both to complete Hi dave13

Basically, If $$\frac{1}{x}$$ of the work takes place in 1 hour, the time taken to complete the work will be x!

Here in this question, x = $$\frac{7}{12}$$

So, when we finish $$\frac{1}{(\frac{7}{12})}$$ of the work in 1 hour, the time taken to complete the work is $$\frac{12}{7}$$

Hope that helps you!
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