Answer:
a) 
The probability can be determined using the complement rule, with the standard normal distribution, an excel sheet, or a calculator.

b) 
This probability can also be calculated using the normal standard distribution, an excel sheet, or a calculator.

c) 
For this one, the probability can likewise be derived from the standard normal distribution, excel, or a calculator, with specific adjustments:

Step-by-step explanation:
Previous concepts
Normal distribution refers to a symmetric probability distribution centered around the mean, indicating that occurrences near the mean are more common than those far from it.
The Z-score is a statistic that represents a value's relationship to the average of a set of values, expressed in terms of how many standard deviations it is away from the mean.
Part a
Let X denote the random variable representing the lengths within a population, and for our case, the distribution for X is as follows:

Where
and 
We seek the probability:

The most effective way to solve this is by leveraging the normal distribution and the corresponding Z-score:

By applying this formula, we can find the probability:

Again, this probability can be obtained either using the complement rule, the standard normal distribution, or a calculator.

Part b

This probability can also be computed using either the normal standard distribution, an excel sheet, or a calculator.

Part c

In this case, the probability can similarly be acquired with the help of the standard normal distribution, an excel sheet, or a calculator, with particular adjustments:
