Finding Sarah's Age

Emily is twice as old as her sister Sarah. If the sum of their ages is 36 years, how old is Sarah?

Let Sarah's age be represented by the variable $x$. Therefore, Emily's age can be represented as $2x$. According to the problem, we can set up the following equation:

$$ x + 2x = 36 $$

From this, we can solve for $x$ to find Sarah's age.