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Monday
0.9 mg/L
Tuesday
1.0 mg/L
Wednesday
0.9 mg/L
Thursday
1.3 mg/L
Friday
1.1 mg/L
Saturday
1.4 mg/L
Sunday
1.2 mg/L
Solution: Add up the seven chlorine residual readings: 0.9 + 1.0 + 0.9 + 1.3 + 1.1 + 1.4 + 1.2. = 7.8.
Next, divide by the number of measurements—in this case, 7:
7.8 ÷ 7 = 1.11
The mean chlorine residual for the week was 1.11 mg/L.
EXAMPLE 2.23
Problem: A water system has four wells with the following capacities: 115 gpm (gallons per min-
ute), 100 gpm, 125 gpm, and 90 gpm. What is the mean?
Solution:
115 gpm + 100 gpm + 125 gpm + 90 gpm = 430
430 ÷ 4 = 107.5 gpm
EXAMPLE 2.24
Problem: A water system has four storage tanks. Three of them have a capacity of 100,000 gal each,
while the fourth has a capacity of 1 million gal. What is the mean capacity of the storage tanks?
Solution: The mean capacity of the storage tanks is
100,000 + 100,000 + 100,000 + 1,000,000 = 1,300,000
1,300,000 ÷ 4 = 325,000 gal
Notice that no tank in Example 2.24 has a capacity anywhere close to the mean.
EXAMPLE 2.25
Problem: Effluent biochemical oxygen demand (BOD) test results for the treatment plant during the
month of August are shown below:
Test 1
22 mg/L
Test 2
33 mg/L
Test 3
21 mg/L
Test 4
13 mg/L
What is the average effluent BOD for the month of August?
Solution:
22 + 33 + 21 + 13 = 89
89 ÷ 4 = 22.3 mg/L
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