Question 774
First, we need to write out what we know as equations:

{{{s = 8+2b}}}
{{{(s+5) + (b+5) = 69}}}

Substituting the first equation into the second gives:

{{{(8+2b+5) + (b+5)=69}}}
{{{18+3b=69}}}
{{{3b=51}}}
{{{b=17}}}

Brad is 17 years old.