Battery life between charges for the Motorola Droid Razr Maxx is 20 hours when the primary use is talk time (The Wall Street Journal, March 7, 2012). The battery life drops to 7 hours when the phone is primarily used for Internet applications. Assume that the battery life for both uses follows an exponential distribution.

a. Show the probability density function for battery life for the Droid Razr Maxx phone when its primary use is talk time.

b. What is the probability that the battery charge for a randomly selected Droid Razr Maxx phone will last no more than 15 hours when its primary use is talk time?

c. What is the probability that the battery charge for a randomly selected Droid Razr Maxx phone will last more than 20 hours when its primary use is talk time?

d. What is the probability that the battery charge for a randomly selected Droid Razr Maxx phone will last no more than 5 hours when its primary use is Internet applications?