First, calculate how much is the discount equal to. Then, substract the discount from the original price to find the selling price.
Since the discount is 10%, find how much is 10% of $15 by multiplying 15 times 10/100:
[tex]15\times\frac{10}{100}=1.5[/tex]Then, 10% of 15 is equal to 1.5. Now, substract 1.5 from 15 to find the final price:
[tex]15-1.5=13.5[/tex]Therefore, the girl will pay $13.5. The answer is:
[tex]\text{ \$13.5}[/tex]