, , and
What is the PF?
Let's start by filling in the values. SO
$\displaystyle 19=9(\frac{1}{1-\frac{Pf}{478}})$
Simplifying
$\displaystyle 19=9(\frac{1}{\frac{478-Pf}{478}})$
$\displaystyle 19=9(\frac{478}{478-Pf})$
$\displaystyle 19=(\frac{4302}{478-Pf})$
After this we cross-multiply. and solve for Pf.