Martina's penny bank is 16 full. after she adds 440 pennies, it is 23 full. how many pennies can martina's bank hold?

In the area that she observed, the water level fell 13.64mm over 6.4 years, for an average annual rate of change of:
-13.64 ÷ 6.2 = -2.2 mm/ year. 

The difference between how much average water levels rose and how much water levels in her part of the river fell is:
1.8 -(-2.2) = 4 mm/year

During the 6.2 year period: 
4 mm/year * 6.2 = 24.8 mm