Aunt Anna is $42$ years old. Caitlin is $5$ years younger than Brianna, and Brianna is half as old as Aunt Anna. How old is Caitlin?
If Brianna is half as old as Aunt Anna, then Brianna is $\frac{42}{2}$ years old, or $21$ years old. If Caitlin is $5$ years younger than Brianna, she is $21-5$ years old, or $16$. So, the answer is $\fbox{16}$