There are 365.25 days a in year. (.25) for the leap year which has 366 days in the year every four years.There are 30 days in May. So to find the probability that one of them was born in May your odds would be 30 opportunities in 365.25 or 30/365.25 = .082135. Basically the odds here are 1/12, with slightly less chance for all the months with 30 days than 31 days, and the least chance for February which only has 28 days. Since we are trying to determine the odds of two people selected at random who would both be born in May the odds would be roughtly 1/12 x 1/12 = 1/144. Or exactly 900/133407.5625 = .006742