Average Height of Plants in Group A

A teacher conducted an experiment to measure the growth of a group of plants over a month. The plants were divided into three groups:

Group A: Received sunlight for 12 hours a day.

Group B: Received sunlight for 8 hours a day.

Group C: Received sunlight for 4 hours a day.

The heights of the plants (in centimeters) after four weeks were recorded as follows:

Group A: 15, 17, 20, 18

Group B: 10, 11, 12, 13

Group C: 5, 6, 7, 7

What is the average height of the plants in Group A?