Answer: Michael pays $12.19 for the headphones including the tax.
Step-by-step explanation:
The initial price of the headphones was $15.50
When Michael uses his coupon, the price is decreased by $4, then the new price will be:
$15.50 - $4 = $11.50
Now Michael must pay 6% for taxes (on the discounted price), then he must pay:
$11.50 + (6%/100%)*$11.50
$11.50 + 0.06*$11.50
$11.50*1.06 = $12.19
Then Michael pays $12.19 for the headphones including tax.