We know that one layer allows 80% of incident radiation to pass and absorbs 100%-80% = 20% of radiation.
Let be the initial amount of radiation. After passing through one layer, 80% of (or ) will remain. This amount of radiation falls onto the second layer, and after passing through it will remain. Therefore, the amount of radiation remaining after passing through layers is Let us determine for which or .
Therefore, the minimal number of layers is 6 and the total thickness will be
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