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Answer:
The time span between 1680 and 1766 (lines 1 and 2) is 86 years: 1766 − 1680 = 86 years. During this time, the falls receded an average of 5.8 feet per year: 500 feet ÷ 86 years ≈ 5.8 feet per year.
The time span between 1856 and 1876 (lines 3 and 4) is 20 years: 1876 − 1856 = 20 years. During this time, the falls receded an average of 25 feet per year: 500 feet ÷ 20 years = 25 feet per year.
Explanation:
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