Question

Whats the best way to do this operation?---------------------------------

Phillip charged $400 worth of goods on his credit card. On his first
bill, he was not charged any interest, and he made a payment of $20. He
then charged another $18 worth of goods. On his second bill a month
later, he was charged 2% interest on his entire unpaid balance. How much
interest was Phillip charged on his second bill?

Answer 1

$400 - $20 = $380 ( The original charge subtract the amount he paid. )
$380 + $18 = $398 (The amount unpaid plus another charge equal total amount unpaid.)
$398 x 0.02 = $7.96 (The amount unpaid multiply the percentage of interest = amount of interest.)

Phillip charged $7.96 interest on his second bill.