Before refill (days 1-13): Add the water level readings from days 1, 4, 7, 10, 13 and divide by 5.<br><br>
After refill (days 16-28): Add the water level readings from days 16, 19, 22, 25, 28 and divide by 5.<br><br>
Difference = Average before refill - Average after refill.<br><br>
Final Answer: Read from the graph and compute.