Need help with this math problem....

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?

okay, so I know the answer is $7.96, but I have no idea how to get that answer... can someone please explain how to get the answer???
thanks! :)

You need to find the unpaid balance.
Started with 400
paid 20, 380
spend 18, 398

398 is the unpaid balance.
2% of this is .02*398=7.96

I would imagine

$400 less the $20 = $380 New Balance - No Charge

$380 + $18 = $398 New Balance or entire balance.

The interest on this entire balance is at 2%. So $398 x 2% or $398 x 0.02 = $7.96