Question 340358
Let {{{X}}} be the number of years until the father is 3 times the son's age,
{{{F=3S}}}
{{{(39+X)=3(15+X)}}}
{{{39+X=45+3X}}}
{{{2X=-6}}}
{{{X=-3}}}
It happened three years ago when the father was 36 and the son was 12.