Question 147811
First set variables for James and Eric.  Since James is 3 times the age of Eric, and 5 years ago James was 4 times the age of Eric, you would set up two equations.  Let's say that Eric is e.  Then your equations would be {{{james=3e}}} and {{{james-5=4(e-5)}}}.  You can use the first equation {{{james=3e}}} and plug in that value for james in the second equation.  Then it becomes {{{3e-5=4(e-5)}}}.  Then distribute on the right side {{{3e-5=4e-20}}}.  Add five to both sides and get {{{3e=4e-15}}}.  Then subtract 4e from both sides and get {{{-e=-15}}}.  cancel out the negatives and you are left with {{{e=15}}}.


Eric=e and James=3e.  Plug in 15 for e and you get Eric=15 and James=45.
To check, subtract 5 from each and get 10 and 40. Since 40 is 10(4), then our ages are correct. Eric=15 and James=45.