The Johnson family uses a propane tank for cooking on their gas grill. During the summer they need to replace their tank on average every 30 days. At a randomly chosen moment, what is the probability...

The Johnson family uses a propane tank for cooking on their gas grill. During the summer they need to replace their tank on average every 30 days. At a randomly chosen moment, what is the probability that they can grill out (a) at least 40 days before they need to replace their tank; (b) no more than 20 days?