what is the probability that a non leap year chosen at random has
a) 52 sundays
b)53 sundays

Part a is obvious.

Part b is the same as asking if a non-leap year starts on a Sunday. There are 7 days in a week. Wouldn't it be 1/7?

Part a is no more obvious than part b. A non leap year has 365 days. Divide by 7 and you see that there are 52 weeks plus one extra day per year. So what is the probability that the extra day is a Sunday (part b), and the probability that the extra day is something other than a Sunday (part A)?