Ordinary year has 365 days
365 days = 52 weeks + 1 day
That 1 day may be Sun, Mon, Tue, Wed, Thu, Fri, Sat
Total no. of possible outcomes = 7
Let E ⟶ event of getting 53 Sundays
No. of favourable outcomes = 1 {Sun}
P(E) = (No.of favorable outcomes)/(Total no.of possible outcomes) = 1/7