Question 533880
Let their ages be {{{a}}} and {{{b}}}
{{{ a + b = 15 }}}
{{{ a - b = 3 }}}
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add the equations
{{{ 2a = 18 }}}
{{{ a = 9 }}}
and, since
{{{ a + b = 15 }}}
{{{ b = 6 }}}
Their ages are 6 and 9