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?

122.9

Explanation

The hourly error rate for this machine is the error rate in parts per 100 multiplied by the number of parts produced per hour: $$\frac{4}{100} \times 8=0.32$$ 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 $$16 \times 7.68 = 122.88$$ approximately 122.9 error free parts were produced yesterday.

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