Aclub has 160 members, some of whom play tennis or cricket or both.90 play tennis or cricket. 10 play neither, x play both tennis n cricket.

X=60 Because 10+90 is 100 and 160-100  is 60, which is the number of players who play tennis and cricket.

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