0.001 times mask is faulty and 0.999 times it is fine
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Part 1. All 1000 masks are effective
${0.999}^{1000}$ is the probability that the first 1000 masks are not faulty
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Part 2. All mask is effective : $0.999^{10000000}$
At least 1 is not effective : 1 – $0.999^{10000000}$