9. The difference between two numbers is 24. Find the numbers if their
sum is 88.

The numbers are 56 and 32

Step-by-step explanation:

Let the two numbers be x and y

x - y = 24

x = 24 + y --- ( 1 )

x + y = 88 --- ( 2 )

Substitute ( 1 ) into ( 2 ):

24 + y + y = 88

2y = 88 - 24

2y = 64

y = 32

Substitute y = 32 into ( 1 ):

x = 24 + 32

x = 56

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