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GMAT Prep 1 #3_PS Rates Work #13

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by kwah » Mon Mar 19, 2012 11:35 am
I have attached a question from GMAT Prep Test 1.

What is the most efficient way to achieve this result?

Answer:C

Please advise, thank you.
K
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by LalaB » Mon Mar 19, 2012 11:58 am
Rate of the 1st machine+ Rate of the 2nd machine=Rate of both machines

(x/36)+(x/18)=1/2
x=6
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by Anurag@Gurome » Mon Mar 19, 2012 8:16 pm
kwah wrote:I have attached a question from GMAT Prep Test 1.

What is the most efficient way to achieve this result?

Answer:C

Please advise, thank you.
K
In 1 hour, machine R does 1/36th of the job

In 1 hour, machine S does 1/18th of the job

In 1 hour, machine R and S together does 1/36 + 1/18 = 3/36 = 1/12th of the job or R + S together take 12 hours to do 1 job.

Now if the job is to be done in 2 hours, then we'll need 12/2 = 6 machines each.

The correct answer is C.
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by GMATGuruNY » Tue Mar 20, 2012 2:54 am
A company has two types of machines, type R and type S. Operating at a constant rate, a machine of type R does a certain job in 36 hours and a machine of type S does the same job in 18 hours. If the company used the same number of each type
of machine to do the job in 2 hours, how many machines of type R were used ?

A) 3
B) 4
C) 6
D) 9
E) 12
Let the job = 36 units.
Rate for machine R = 36/36 = 1 unit per hour.
Rate for machine S = 36/18 = 2 units per hour.
To complete the job in 2 hours, the number of units produced each hour = 36/2 = 18 units.
Since using one of each machine will produce 1+2 = 3 units per hour, and 18 units must be produced, we need 18/3 = 6 of each machine.

The correct answer is C.
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by [email protected] » Tue Mar 20, 2012 6:14 am
simple, but the question confused me to think whether they only want it to be tested with R machines or with the combination of (R+S)..later i understood that they are asking us to calculate first with both and then ask how much was R in that....good one again
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by dhiren8182 » Wed Mar 21, 2012 5:53 am
Can we used the direct formula:

rates of r*rate of y/r+y
36*18/54=12 in 1 hr
so in 2 hrs 6 machines will be required.
Ans is C
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by ronnie1985 » Wed Mar 21, 2012 9:09 am
Let x be the required no of machines each. rate of work = x/36+x/18 per hour. Required rate = 1/2 per hour. Therefore, 3x/36 = 1/2 => x = 6. Six machines each required.
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