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## Two models of computers

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metallicafan Really wants to Beat The GMAT!
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Two models of computers Tue Apr 24, 2012 10:08 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
A company has two models of computers, model M and model N. Operating at a constant rate, a model M computer can complete a certain task in 18 minutes and a model N computer can complete the same task in 9 minutes. If the company used the same number of each model of computer to complete the task in 1 minute, how many model M computers were used

(A) 2
(B) 3
(C) 6
(D) 9
(E) 12

Source: www.gmathacks.com

OA: C

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Shalabh's Quants Really wants to Beat The GMAT!
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Tue Apr 24, 2012 10:43 am
metallicafan wrote:
A company has two models of computers, model M and model N. Operating at a constant rate, a model M computer can complete a certain task in 18 minutes and a model N computer can complete the same task in 9 minutes. If the company used the same number of each model of computer to complete the task in 1 minute, how many model M computers were used

(A) 2
(B) 3
(C) 6
(D) 9
(E) 12

Source: www.gmathacks.com

OA: C
Comp M & N's 1 min. job = 1/18+1/9 = 1/6.

It means to complete a job, they need 6 mins. Or to do it 1 min. M & N should be 6 each.

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Last edited by Shalabh's Quants on Wed Apr 25, 2012 12:00 am; edited 1 time in total

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tpr-becky GMAT Instructor
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Tue Apr 24, 2012 10:19 pm
Rate problems that do not give a specific amount for the job can best be solved by picking a number for the job so you don't have to work with fraction. In this case you can pick 36 becuase it divides easily into both 9 and 18.

The rate formula is D=R(t) and you often have to use this formula multiple times for different scenarios within a problem.

The first information we get is that M can do the job in 18 minutes that means that 18(M)=36 - thus the rate of M=2.

Next we get that N can do the job in 9 minutes that means 9(N)=36 - the rate of N=4

Then it asks if you are using the same number of each machine and you finish in one minute how many of each machine.

the task stays at 36 because it hasn't changed and the time we are looking for is 1. when machines work together you have to add their rates within the rate formula but here we have some number of each machine thus the combined rate is going to be the sum of that number times each rate.

This gives you the formula: (x(2)+x(4))(1)=36 - which can be solved to 6x=36 thus x=6 and there are 6 of each type of machine, thus the answer is C.

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redng Just gettin' started!
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Tue Apr 24, 2012 11:05 pm
M's rate = 1/18
N's rate = 1/9
x = number of machines

(1/18x + 1/9x) * (1min) = 1task
1/6x = 1
x = 6

C

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Wed Apr 25, 2012 12:12 am
M's rate = 1/18 in 1 minute
N's rate = 1/9 in 1 minute

lets say x number of each m and n machine were used:
therefore, work done in 1 min is:

X*(1/18 + 1/9)
Since all the work is done in 1 min

X*(1/18 + 1/9) = 1
Solving for X, X = 6, therefore 6 machines of M and N each are used.

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Stuart Kovinsky GMAT Instructor
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Wed Apr 25, 2012 12:27 am
metallicafan wrote:
A company has two models of computers, model M and model N. Operating at a constant rate, a model M computer can complete a certain task in 18 minutes and a model N computer can complete the same task in 9 minutes. If the company used the same number of each model of computer to complete the task in 1 minute, how many model M computers were used

(A) 2
(B) 3
(C) 6
(D) 9
(E) 12

Some good solutions posted already; we can also use the basic work formula for two workers:

CT = combined time
A = time for worker 1 to do job on its own
B = time for worker 2 to do job on its own

CT = (A*B)/(A+B)

In this case:

CT = 9*18/(9+18) = 162/27 = 6

Since it takes 6 minutes for 1 of each machine to complete the job, we need 6 of each machine to complete the job in 1 minute: choose (C)!

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Wed Apr 25, 2012 2:43 am
Model M Computer's Rate = 1/18

Model N Computer's Rate = 1/9

Combined they do it in; 1/18 + 1/9

Actually the equation goes to: 1/18X n1 + 1/9 X n2 = 1

where n1 and n2 are the no.of machines or computers being used of each model i.e M and N

It is specifically given in the question, that the number of computers being used for both the models are the same i.e

n1 = n2

so now the equation turns out to be

n1/18 + n1/9 =1

therefore, n1/18 + 2n1/18 = 1

therefore, 3n1 = 18

therefore, n1 = 6

and so is n2

I hope this really helped to solve the works problem effectively...

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