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287. Three machines, individually, can do a certain job in 2, 3 and 4 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?
289. In an examination of 4 subjects, John gets 100, 60 and 55 in three subjects. If his average (arithmetic mean) marks for all four subjects is 75, then the marks in the 4th subject must be