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Purely theoretical (in practice sunscreen isn't as strong as it says on the bottle to begin with) you multiply it. If 1/3rd of the UV photons make it through the first layer, and of those that mad...
Answer
Purely theoretical (in practice sunscreen isn't as strong as it says on the bottle to begin with) you multiply it. If 1/3rd of the UV photons make it through the first layer, and of those that mad...