The defect length of a corrosion defect in a pressurized steel pipe is normally distributed with mean

Question:

The defect length of a corrosion defect in a pressurized steel pipe is normally distributed with mean value 30 mm and standard deviation 7.8 mm [suggested in the article "Reliability Evaluation of Corroding Pipelines Considering Multiple Failure Modes and Time Dependent Internal Pressure" (J. of Infrastructure Systems, 2011: 216-224)].
a. What is the probability that defect length is at most 20 mm? Less than 20 mm?
b. What is the 75th percentile of the defect length that is, the value that separates the smallest 75% of all lengths from the largest 25%?
c. What is the 15th percentile of the defect length What values separate the middle 80% of the defect length from the smallest 10% and the largest 10%?
Distribution
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