Probability-and-statistics

(a) Two friends agree to meet at a park with the following conditions. Each will reach the park between 4:00 p.m. and 5:00 p.m. and will see if the other has already arrived. If not, they will wait for 10 minutes or the end of the hour whichever is earlier and leave. What is the probability that the two will not meet?

(b) Given a regular expression for the set of binary strings where every 0 is immediately followed by exactly k 1’s and preceded by atleast k 1’s (k is a fixed integer).