The number of years it will take until the stalactite is 1 meter long is; 70 years
From conversion units, we know that;
1 m = 100 cm
1 cm = 100 m
Current length is given as 72 cm
Thus;
Length after x years = 1 m = 100 cm
Length increase in x years = 100 -72 = 28 cm
Since 1 cm = 10 mm
Then;
Length increase in x years in mm = 28 * 10 = 280 mm
Thus;
Length increase in 1 year = 4 mm
Length increase in x years = 4x mm
Then; 4x = 280
x = 70
Thus, it will take 70 years until it is 1 meter long.
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