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

(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.