The regular price is $9.99. A 60% markup of that would be 0.60($9.99) = $5.99. You'll then have to add this markup ($5.99) to the regular price ($9.99).
A faster way to calculate the selling price would be to mult. the regular price ($9.99) by 1.60 (which includes both the reg. price and the markup):
1.60($9.99) = $15.98 (selling price)