If Jeremiah walks ½ mile in fifteen minutes, calculate the rate of miles he walks in an hour using a complex fraction.

There are 3 green marbles in the box.

Step-by-step explanation:

15% = 0.15

To figure out how much 15% of 21 is, you must multipl the decimal form of the percentage by the whole number. This will give you how much of that number that percentage is.

0.15 * 21 = 3.15  

But you can't have 0.15 of a marble, so you must round as well.

3 green marbles in the box.