grandh01 wrote: ↑Mon Aug 13, 2012 7:31 pm
Three machines, individually, can do a
certain job in 4, 5, and 6 hours,
respectively. What is the greatest part
of the job that can be done in one hour
by two of the machines working
together at their respective rates?
(A)11/30
(B)9/20
(C)3/5
(D)11/15
(E)5/6
Assign a "nice" value to the job (i.e., a value that works well with the given numbers, 4, 5 and 6)
Since 4, 5 and 6 all divide into
60, let's say that completing the job means making
60 widgets
So, the first machine can make
60 in 4 hours, which means its rate is
15 widgets per hour.
The second machine can make
60 in 5 hours, which means its rate is
12 widgets per hour.
The 3rd machine can make
60 in 6 hours, which means its rate is 10 widgets per hour.
So, the first machine and second machine are the two fastest.
Their
combined rate =
15 +
12 =
27 widgets per hour
So, in one hour, those two machines can make
27 widgets, which means they can complete
27/
60 of the job.
27/
60 simplifies to be 9/20
Answer: B
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