6. You have an ofce phone for work and a cell phone for personal use. The amount of time X before the ofce phone rings has an exponential distribution with mean 30 minutes. The amount of time Y before the cell phone rings has an exponential distribution with mean 10 minutes. The two random variables X and Y are independent. (a) Determine the joint probability density function of the two amounts of time. (b) Determine (Cov(X, Y). (c) What is the probability that the work phone rings before the personal phone