A half-life refers to the amount of time it takes for half of a substance to degrade. Half-life is commonly used in regard to radioactive material because it take a long time for radioactive material to become safe for humans. The half-life varies by substance, with some substances having relatively shorter half-lives and others having very long half-lives. To find the quantity of a substance remaining, you need to know the time elapsed and the amount you started with.
Instructions
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1
Divide the time elapsed by the period of one half-life. For example, if 20 years elapsed and one half-life equals 5 years, divide 20 by 5 to find 4 half-lives have passed.
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2
Raise 0.5 to the power of number of half-lives elapsed. In this example, raise 0.5 to the fourth power to get 0.0625.
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3
Multiply the result by the original quantity to find the remaining quantity. For example, if you started with 20 lbs., multiply 20 by 0.0625 to get 1.25 pounds remaining after four half-lives.