The Johnson family uses a propane gas grill for cooking outdoors. During the summer they need to

Question:

The Johnson family uses a propane gas grill for cooking outdoors. 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?