There are two ways you could answer this question, depending on whether or not you have a calculator:
1. (with a calculator) You do 39.99 × 0.3 = 11.997.
Then, 39.99 - 11.997 = $27.993, or $27.99.
2. (without a calculator). You have to figure out 30% of 39.99, so you divide it by 10 to get 10%, which is 3.999, and then multiply by 3 to get 30%, which is 11.997. Then, same as in the first way, you do 39.99 - 11.997 = $27.99.
Hope this helps!