Lost in stats!

#1
The amount of time one spends in a bank is exponentially distributed with mean 10 minutes.
a.) What is the probability that a customer will spend more than 15 minutes in the bank?

b.) What is the probability that a customer will spend more than 15 minutes in the bank given that she is till in the bank after 10 minutes?


I do not have a clue how to begin this problem! I am not even sure what mean is!!!!

Can anyone help?