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To determine how many total machine hours are required to finish the job, it's important to understand the relationship between the number of machines, the amount of time they work, and the total machine hours.

In this scenario, 9 machines are working for 5 hours. To find the total machine hours, you can multiply the number of machines by the number of hours they operate. This means:

Total machine hours = (Number of machines) × (Hours worked)

Substituting the numbers provided gives:

Total machine hours = 9 machines × 5 hours = 45 total machine hours.

This calculation illustrates that to finish plowing the field, 45 machine hours are necessary, which accounts for the full contribution of all machines working together over the specified time. Managing the efficiency and utilization of the machines directly leads to this figure representing the overall effort needed for the task at hand.