Question 696182
This is a tricky word problem.

Here is how you set it up.
Let M = Mary's age
Let A = Ana's age
Let Y = # of years

Equation 1: {{{M = 24}}} (From mary is 24 years old)
Equation 2: {{{M - Y = A}}} (From when mary was as old as ana is now)
Equation 3: {{{M = 2*(A - Y)}}} (From mary is twice as old as ana was)
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We know the value of M so we can solve equation 2 for A
{{{M - Y = A}}}
{{{24 - Y = A}}}
Now plug 24 - Y into equation 3 for A, and M= 24
Equation 3: {{{M = 2*(A - Y)}}}
{{{24 = 2*((24 - Y)-Y)}}}
Simplify the equation
{{{24 = 2*(24 - 2Y)}}}
Simplify
{{{24 = 48 - 4Y}}}
Add 4Y to both sides
{{{24 + 4Y = 48}}}
Subtract 24 from both sides
{{{4Y = 24}}}
Divide both sides by 4
{{{highlight(Y = 6)}}}
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Now we can solve equation 2 to find Ana's age
Equation 2: {{{M - Y = A}}}
{{{24 - 6 = A}}}
{{{highlight_green(18 = A)}}}