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What is the thickness of lead required to reduce the intensity of a gamma ray beam with an initial intensity of 10^6 Bq to 100 Bq after passing through it? Assume that the gamma rays have an energy of 1 MeV and that the mass attenuation coefficient of lead for this energy is 0.13 cm^2/g.
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<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

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<|begin_of_solution|>

The answer is 42.

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