Question 388031
"Four years ago, Lisa was twice as old as John"
{{{L-4=2(J-4)}}}
{{{L-4=2J-8}}}
1.{{{L=2J-4}}}
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"Four years from now, John will be three quarters of Lisa's age."
{{{J+4=(3/4)(L+4)}}}
{{{4(J+4)=3(L+4)}}}
{{{4J+16=3L+12}}}
2.{{{4J=3L-4}}}
Substitute from eq. 1 into eq. 2,
{{{4J=3(2J-4)-4}}}
{{{4J=6J-12-4}}}
{{{-2J=-16}}}
{{{highlight(J=8)}}}
Then from eq. 1,
{{{L=16-4}}}
{{{highlight(L=12)}}}
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Right now, Lisa is 12 and John is 8.