What is the proportion
It is estimated that the mean life span of concrete drill bits is 20 hours. Suppose a construction company purchases drill bits that have a lifespan that is approximately normally distributed with a mean equal to 20 hours and and a standard deviation of 2 hours.
- What is the proportion of the company’s drill bits will fail before 18 hours of use?
- What is the proportion will last at least 18 hours?
- What proportion will have to be replaced after more than 25 hours of use?