Normal distribution

Hey guys, Started studying normal distribution recently and would really appreciate any help with the following questions:

• Assume that the waiting time of patients in a doctor’s waiting room is normally distributed with a mean of 20 minutes and a standard deviation of 4 minutes. What is the probability that a patient (who is going to be randomly selected) will need to wait exactly 19 minutes? Justify your reasoning. Your answer should include an appropriately defined random variable (Someone told me it's supposed to be 0 but I don't get why)

• "Give a reason as to why it may not be sensible to assume that a random variable which can only take positive values has a normal distribution."

Thank you!