Question 583217
Let {{{ c }}} = number of children
Let {{{ a }}} = number of adults
given:
{{{ a + c = 68 }}}
{{{ c = 3a }}}
---------
{{{ a = c/3 }}}
{{{ c/3 + c = 68 }}}
{{{ c + 3c = 204 }}}
{{{ 4c = 204 }}}
{{{ c = 51 }}} 
There are 51 children
check:
{{{ a + c = 68 }}}
{{{ a + 51 = 68 }}}
{{{ a = 17 }}}
and
{{{ c = 3a }}}
{{{ 51 = 3*17 }}}
{{{ 51 = 51 }}}
OK