What is the chance that an ordinary year selected at random contains 53 Sundays?
An "ordinary" year has 365 days.
Note that: .
. . That is, a year has 52 full weeks plus one day.
This means that a year begins on a certain weekday
. . and also ends on the same weekday.
Hence, a year starting on Sunday will have 53 Sundays.
Assuming the starting day for a year is evenly distributed,
. . the probability of beginning on a Sunday is