My guess would be she invests $\$$x per year, so her living wage would be $\$$47,000-x. So, my guess would be, at an annual percentage yield of $n$%, she would want to save

\(\displaystyle 47000\dfrac{\left(1+\dfrac{n}{100}\right)^{20}-1}{\left(1+\dfrac{n}{100}\right)^{20}+\left( 1 + \dfrac{n}{100} \right)^{62}-2}\)

per year. If there is no interest, this becomes a simpler calculation:

$42x = 20(47000-x)$

which gives

$x \approx 15161$

So, her "comfortable living" would be $\$31839$.