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