Question 720854
The difference in their ages does not change
Let {{{ a }}} = Mark's age now
Let {{{ b }}} = Michelle's age now
{{{ a = b + 20 }}}
In 4 years:
{{{ a + 4 = 2*( b + 4 ) }}}
By substitution,
{{{ b + 20 + 4 = 2b + 8 }}}
{{{ b = 24 - 8 }}}
{{{ b = 16 }}}
and, since
{{{ a = b + 20 }}}
{{{ a = 16 + 20 }}}
{{{ a = 36 }}}
Mark's age now is 36
Michelle's age now is 16
check:
in 4 years, they will be 40 and 20
Mark is twice as old
OK