Question 841378
Let {{{ f }}} = the father's age
I can say:
{{{ 12 = (1/3)*f }}}
{{{ f = 3*12 }}}
{{{ f = 36 }}}
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The difference in their ages does not change
It is {{{ 36 - 12 = 24 }}}
So, when the boy is 24 the father is 
{{{ 24 + 24 = 48 }}}