Probability Density Function

My college proffesor says its fine to use the calculator to find the answer so for a and b I did.. I got 20 and 13.862. However I want to learn to do these problems by hand.. for c and d Im stuck.. Thank you for your help.

Suppose that p(x) = 0.05e^(-0.05x), where x is greater than 0, is a probability density function for the number of people in line at a movie theater.

a. Find the mean number of people in line and interpret.

b. Find the median number of people in line and interpret.

c. Find the probability that there are between 10 and 20 people in line

d. Find the probaiblity that there are more than 30 people in line.