The Radioisotope Potassium-40 Decays to Argon-40 by Positron Emission with a Half-Life

question 55

Multiple Choice

The radioisotope potassium-40 decays to argon-40 by positron emission with a half-life of 1.3 * 10⁹ yr. A sample of moon rock was found to contain 78 argon-40 atoms for every 22 potassium-40 atoms. The age of the rock is

• A) 8.1 * 10^-10 yr.
• B) 2.4 * 10⁹ yr.
• C) 2.8 * 10⁹ yr.
• D) 4.6 * 10⁹ yr.
• E) 6.8 * 10⁹ yr.

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