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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

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sqdancefan

Answer:

  (B)  9/20

Step-by-step explanation:

The fastest machine can do 1/4 of the job in 1 hour. The second-fastest machine can do 1/5 of the job in 1 hour. Together, these two machines can do ...

  (1/4) +(1/5) = (5+4)/(5·4) = 9/20

of the job in 1 hour.

9/20 of the job can be done in 1 hour by two of the machines.

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