Question 598924
Let {{{ n }}} = number of years when Father is 
twice as old as Daughter
{{{ 42 + x = 2*( 8 + x ) }}}
{{{ 42 + x = 16 + 2x }}}
{{{ x = 42 - 16 }}}
{{{ x = 26 }}}
In 26 years the Father will be twice as old
check:
Father will be {{{ 42 + 26 = 68 }}}
Daughter will be {{{ 8 + 26 = 34 }}}
{{{ 68 = 2*34 }}}
OK