Question 128175
m is Matthew's age now.  j is Jenny's age now.


{{{m=3j}}} because Matthew is three times as old as Jenny now.


In 7 years, Matthew will be {{{m+7}}} years old, and Jenny will be {{{j+7}}}, and at that time:


{{{m+7=2(j+7)}}} because Matthew will be twice as old as Jenny in 7 years.


But we know that {{{m=3j}}} so we can substitute this value into our last equation:


{{{cartoon(red(m)+7=2(j+7),red(3j)+7=2(j+7))}}}


{{{3j+7=2(j+7)}}}
{{{3j+7=2j+14}}}
{{{3j-2j=14-7}}}
{{{j=7}}}


So Jenny is 7 now, and Matthew is {{{3*7=21}}}.


Check:
In seven years Jenny will be 14 and Matthew will be 28.  28 = 2 * 14.  Answer checks.