It is important to know that July has 31 in total.
The probability of being born in July is
[tex]P=\frac{1}{12}[/tex]The probability of being born on a day that is multiple of 3 is
[tex]P=\frac{9}{31}[/tex]Because there are 9 days which are multiple of 3.
Then, we sum both probabilities.
[tex]P=\frac{1}{12}+\frac{9}{31}=\frac{31+108}{372}=\frac{139}{372}\approx0.37[/tex]