A hat contains a number of cubes: 15 red, 10 white, 5 blue, and 20 black. One cube is chosen at random. What is the probability that it is?
a. A red cube?
b. Not a red cube?
c. A cube that is white OR black?
d. A cube that is neither white nor black?
e. What do the answers to part a and part b add up to and why?