13^th day is going to be any of the day from Monday to Sunday. So probability that 13^th day of a month being Friday is 1/7.

But here the month is not declared. So the probability of selecting a random month is 1/12

By multiplication theory of Probability -

∴ probability that the 13^th days of a randomly chosen month is Friday = (1/7)×(1/12) = 1/84