To find the amount that Franklin financed, you will calculate the amount he put down and subtract that from the original price because that is what he still owes.
0.2 x $4500 = $900
$4500 - $900 = $3600.
Porter financed $3600.
You also multiply 0.8 x $4500 because this represents the part out of 100% he still owes. You will get the same answer this way.