Answer:

And then we can calculate the probability of interest using the complement rule:

Explanation:
Let's assume that the random variable X ="Lengths of her classes", and we know that the distribution for X is given by:

The expected value is given by:

And the variance given by:

The density function is given by:

And 0 for other case.
And on this case we want to find the following probability
first we need to calculate
, so we can calculate this with the following integral:

And after evaluate using the fundamental calculus theorem we got:

And then we can calculate the probability of interest using the complement rule:
