Question 714742
Use TWO variables.  The problem description gives ways to relate the ages of the two people.  


Let p = paul's age 5 years ago
Let e = erin's age 5 years ago


We do not know either of their ages.  NOW is ADDITIONAL 5 years, which is why you find some use of addition in the "NOW" column.


PEOPLE _______________NOW_________________5 Years Ago
Paul__________________{{{(p+5)=2(e+5)}}}________{{{p=3e}}}
Erin_____________________{{{e+5}}}______________{{{e}}}


The table shows, as taken directly from the description and assignment of variables, TWO equations and TWO unknown variables.  Our system to begin with is obviously this:


SYSTEM:--------------
{{{p+5=2(e+5)}}}
{{{p=3e}}}
---------------------


WHAT TO DO NEXT
Using the second equation to substitute for p into the first equation is a simple matter.  {{{p+5=2(e+5)}}}, {{{(3e)+5=2(e+5)}}}, 
{{{3e+5=2e+10}}}
{{{3e-2e=10-5}}}
{{{highlight(e=5)}}}, 5 years ago.


and it follows that p=3e=3(5)=15, {{{highlight(p=15)}}}, 5 years ago.