Question 1156730

 john is {{{x}}} and liza is {{{y}}}

if the sum of john and liza's age is {{{21}}}, we have 

{{{x+y=21}}}

{{{x=21-y}}}.....eq.1

if john is {{{3 }}}years {{{younger}}} than liza, we have

{{{x=y-3}}}.........eq.2, substitute {{{x}}} from eq.1

{{{21-y=y-3}}}........solve for {{{y}}}

{{{21+3=y+y}}}

{{{24=2y}}}

{{{y=12}}}

go to

{{{x=21-y}}}.....eq.1, substitute {{{y}}

{{{x=21-12}}}

{{{x=9}}}

answer: ({{{9}}},{{{12}}}) where john is {{{x}}} and liza is {{{y}}}