Question 161125
Let's start here.
They are 20 miles apart to begin with at 9:30 AM.
Andrew starts riding at 9:30AM.
At 10:00 AM, Andrew has already moved 6 miles (12mph x 1/2hour), so then they are only 14 miles apart. 
This is the distance they have to travel together to meet. 
Andrew moves at {{{12T}}} and Stacy moves at {{{16T}}} to cover the 14 mile gap.
{{{12T+16T=14}}}
{{{28T=14}}}
{{{T=1/2}}}
They both travel an additional 1/2 hour and meet at 10:30AM.
Andrew will have traveled 6+(12*1/2)=6+6=12 miles.
Stacy will have traveled 16(1/2)=8 miles.