Let (F, M, C) be a triple where each of F, M and C is a number from 1 to 365 (birthdays of mother, father, and child, respectively). Then the total number of such triples is 365^{3}. The number of triples where each number corresponds to the 7th of some month is 12 * 11 * 10. Namely, there are 12 variants for mother; for each of those there are 11 variants for father; and for each of the first two there are 10 variants for child. If every triple (F, M, C) is equally likely, then the probability is the ratio .

Note that this probability corresponds to a given day of the month (7th or any other fixed date <= 28). The probability that all three have birthdays on the same day of the month in different monthswithout specifying which day it iswill be of course greater (by about a factor of 30).