It is 1. Every month contains 28 days.
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First we don't consider leap year There is 1 month with 28 days: february There are 4 months with 30 days: april, june, september, november So the probability to select a month with 28 or 30 days is (1+4)/12 = 5/12 If the year is a leap year then the probability is 4/12
Absolute certainty
If you look at the link below, the probability of a 28 year old female having a baby with Down syndrome is a little less than 1 in 1000.
28/52
(25-21) / (28-20) = 4/8 =0.5 or 50%