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The first decay products of potassium-40 are argon-40 and calcium-40.
By subtracting 19 (atomic number of Potassium) from 40. You should come out with 21, which is the number of neutrons in potassium-40
The half life of potassium 40 is 1.3 billion years.
potassium 40, Argon 40, Calcium 40.