The value of c that separates the best 10% of processing times from the remaining 90% is the Z-score corresponding to the 90th percentile of a standard normal distribution, which is approximately 1.28.
The value of c that separates the best 10% of all processing times from the remaining 90% is typically found using a standard normal distribution table or Z-score table. The best 10% would correspond to the 90th percentile, which means we are looking for the Z-score that has 90% of the distribution below it.
In standard normal distribution, the Z-score for the 90th percentile is close to 1.28. Therefore, option a. c = 1.28 is the correct answer. This Z-score value means that processing times faster than c correspond to the top 10% of all processing times when the processing times are normally distributed.