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Larry can build a model car in 3 hours, Moe can build the

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Larry can build a model car in 3 hours, Moe can build the

by AAPL » Wed May 02, 2018 6:43 am

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Larry can build a model car in 3 hours, Moe can build the same car in 4 hours, and Fred can build the car in 5 hours. If the two fastest boys work together, how much of the car can they build in one hour?

A. 1/12
B. 2/7
C. 7/12
D. 8/15
E. 9/20

The OA is C.

We can say the car has 60 parts (lcm of 3, 4 and 5) in all...
L can do 60 parts in 3 hours means 20 parts/hour
Similarly, M can do 15 parts/hour
And F can do 12 parts/hour.

So the L + M (the fastest boys working together) can do 20 + 15 = 35 parts/hour

So, 35/60 = 7/12 in an hour. Option C.

Has anyone another strategic approach to solve this PS question? Regards!
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by Brent@GMATPrepNow » Wed May 02, 2018 8:32 am
AAPL wrote:Larry can build a model car in 3 hours, Moe can build the same car in 4 hours, and Fred can build the car in 5 hours. If the two fastest boys work together, how much of the car can they build in one hour?

A. 1/12
B. 2/7
C. 7/12
D. 8/15
E. 9/20
Your approach (of assigning a nice value to the job) is perfect.
Here's another approach....

RULE: If a person can complete an entire job in k hours, then in ONE HOUR, the person can complete 1/k of the job
Example: If it takes Sue 5 hours to complete a job, then in ONE HOUR, she can complete 1/5 of the job. In other words, her work rate is 1/5 of the job per hour

Larry can build a model car in 3 hours, Moe can build the same car in 4 hours, and Fred can build the car in 5 hours.
So, from the above rule, we can conclude that:
Larry can complete 1/3 of the job in ONE HOUR
Moe can complete 1/4 of the job in ONE HOUR
Fred can complete 1/5 of the job in ONE HOUR

If the two fastest boys work together, how much of the car can they build in one hour?
Since Larry and Moe can each complete more of the job (than Fred can) in ONE HOUR, we know that Larry and Moe are the two fastest boys
In ONE HOUR Larry completes 1/3 of the job
In ONE HOUR Moe completes 1/4 of the job
So, working together, they can complete (1/3 + 1/4 ) of the job
1/3 + 1/4 = 4/12 + 3/12 = 7/12

Answer: C

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by Jeff@TargetTestPrep » Thu May 03, 2018 3:31 pm
AAPL wrote:Larry can build a model car in 3 hours, Moe can build the same car in 4 hours, and Fred can build the car in 5 hours. If the two fastest boys work together, how much of the car can they build in one hour?

A. 1/12
B. 2/7
C. 7/12
D. 8/15
E. 9/20
Larry and Moe are the two fastest boys. Larry's rate is 1 car in 3 hours, or 1/3 car per hour. Moe's rate is 1 car in 4 hours, or 1/4 car per hour. Thus, in one hour, Larry and Moe can build 1/3 + 1/4 = 4/12 + 3/12 = 7/12 of a car.

Answer: C

Jeffrey Miller
Head of GMAT Instruction
[email protected]

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