Question

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

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.