Anvi measured the height of five plants over a three-week experimental period and obtained the following data for the final week (in cm):

To calculate the final average height of the plants, you need to add all the heights together and then divide by the number of plants.

The heights of the plants are: 15 cm, 18 cm, 21 cm, 19 cm, and 20 cm.

1. Add the heights:
   \( 15 + 18 + 21 + 19 + 20 = 93 \)

2. Divide by the number of plants (5):
   \( \frac{93}{5} = 18.6 \)

So, the final average height for this group of plants is 18.6 cm.