Question 709493
Mary is 9 years older than Seth.
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So Seth is x years old and Mary is x+9.
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In a year she will be 3 times as old as he is. 
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In 1 year she will be x+9 + 1, and he will be x + 1,

So x+9 + 1 equals 3 times x + 1

x+9+1 = 3(x+1)

 x+10 = 3x+3
  -2x = -7
    x = {{{3&1/2}}}

So Seth is {{{3&1/2}}}  and Mary is 9 years older or {{{12&1/2}}}.

Checking:

In one year she will be {{{13&1/2}}} and Seth will be {{{4&1/2}}}

So we need to check to ss if {{{13&1/2}}} is three times {{{4&1/2}}}

{{{4&1/2*3 = expr(9/2)*3}}} = {{{27/2=13&1/2}}}

So it checks.  That's a surprise because usually age problems 
have answers which are a whole number of years.  But not in this case.

Edwin</pre>