Franklin porter purchased a waterbed for $4,500. he made a 20 percent down payment and financed the remainder. what amount did he finance?

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.