Question 1167373
What?  "by all but three years"?


Mary is twice as old as Tom:
{{{m=2t}}}


Mary is seventeen:
{{{m=17}}}


Put these two things together.
{{{2t=17}}}
{{{t=17/2}}}
{{{t=8&1/2}}}-------------you can do this only if the situation allows for half-year increments.  Tom is 8 years 6 months.


If you meant or expected something else, then recheck your problem description.