Question 919158

let Brett's age be {{{x}}}
his sister’s age {{{y}}}

given:
Brett is twice as old as his sister. 

{{{x=2y}}} .............1

four years ago he was four times as old as his sister

{{{x-4=4(y-4)}}} .............2


solve the system:
{{{x=2y}}} .............1
{{{x-4=4(y-4)}}} .............2
__________________________________substitute {{{x}}} from 1 in 2

{{{2y-4=4(y-4)}}} .............2..solve for {{{y}}}

{{{2y-4=4y-16}}}

{{{16-4=4y-2y}}}

{{{12=2y}}}

{{{12/2=y}}}

{{{highlight(y=6)}}}...........his sister’s age now


{{{x=2y}}} .............1..plug in {{{6}}} for {{{y}}}

{{{x=2*6}}}

{{{x=12}}} ...........Brett's age now