[InMemoriam]

Your way multiplies kg of K-40 times the percentage of K-40 in the total potassium (dimensionally incorrect).

With dimensions:

0.1 kg K-40 * 1.00 total K
————————————— = 1000 kg total K
0.0001 K-40

[ColdWater]

0.1 kg K-40 * 1.00 total K
————————————— = 1000 kg total K
0.0001 K-40

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I am dimensionally correct.

0.1 kg K-40 .0001 kg K-40 (radioactive)
——— * ———————————— =
body kg k-40