Answer:
21.69% decrease.
Explanation:
We want to determine the percent change from 16.6 to 13.
First, since 13 is smaller than 16.6, we know that this is decrease.
Percent change is given by the formula:

The new value is 13 and the old value is 16.6. Substitute:

Evaluate:

Use a calculator:

We ignore the negative because the negative simply tells us that it's decreasing. (Likewise, if the answer's positive, it's an increase.)
In conclusion, the percent change is an approximate 21.69% decrease.