Let's assume that being born each day has the same probability.
Since there are 7 days in one week, the probability that the person was NOT born on a Monday means that the person was born on any of the other 6 days.
Then
[tex]P(\text{NotMonday)}=\frac{d\text{ays on which the person }could\text{ have been born}}{\text{total number of days in a week }}[/tex]Then,
[tex]\begin{gathered} P(\text{NotMonday)}=\frac{d\text{ays on which the person }could\text{ have been born}}{\text{total number of days in a week }} \\ \downarrow \\ P(\text{NotMonday)}=\frac{6}{7}\cong0.857 \end{gathered}[/tex]We multiply it by 100 in order to transform into a percentage:
0.857 = 0.857 x 100% = 85.7%
