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A machine in a factory has an error rate of 4 parts per 100. The machine normally runs 24 hours a day and produces 8 parts per hour. Yesterday the machine was shut down for 8 hours for maintenance. How many error-free parts did the machine produce yesterday?

A 122.9

The hourly error rate for this machine is the error rate in parts per 100 multiplied by the number of parts produced per hour: $5623_w111_h38.png$ errors per hour
So, in an average hour, the machine will produce 8 - 0.32 = 7.68 error free parts.
The machine ran for 24 - 8 = 16 hours yesterday so you would expect that 16 x 7.68 = 122.88 approxiamtely 122.9 error free parts were produced yesterday.