I don't know how to solve this problem:
Solve for r:
S = L - rL
Thank you and please show your work/explain steps.
$\displaystyle S = L - rL$
take L from both sides
$\displaystyle S -L= - rL$
divide -L from both sides
$\displaystyle \frac{S -L}{-L}= r$
and you are done.
You may want to neaten the LHS
$\displaystyle \frac{L-S}{L}= r$
$\displaystyle 1-\frac{S}{L}= r$