Answer :
You pay $360.
$50 x 4 = $200.
10% of $200 = $20.
$50 x 8 = $400.
10% of $400 = $40.
$400 - $40 = $360
$50 x 4 = $200.
10% of $200 = $20.
$50 x 8 = $400.
10% of $400 = $40.
$400 - $40 = $360