Siblings Ages

Laura is $21 - 12 = 9$ years older than Jason, so Laura was twice as old as Jason when Jason was 9 years old, which was $12 - 9 = \boxed{3}$ years ago.

$L-x=2(J-x)$

$21-x=2(12-x)$

$21-x=24-2x$

$24-21=2x-x$

$\boxed{3=x}$

Write this table:

Laura 21 20 19 18 17 16 15 14 13 12 11 10

Jason 12 11 10 9 8 7 6 5 4 3 2 1

And find the nearest to the left two numbers that you can divide. The nearest is when Laura was 18 and Jason - 9 which was three(3) years ago.

Try Making a Table with Each Year to Simplify the Problem