Before fleeing the country, you find one more folder of 40 documents. Of these,

Question

Before fleeing the country, you find one more folder of 40 documents. Of these,

3 are highly classified (illegal military operations), 17 are merely secret (diplomatic cables), and 20 are

completely mundane (your grandmother’s recipes for biscuits). You randomly choose 3 of these documents

to release.

(a) What is the probability that all of the documents you release are mundane?

(b) What is the probability that none of the doucments you release are mundane?

(c) What is the expected number of mundane documents released?

Explanation / Answer

A) P(realising all mundane) = 20C3 / 40C3 = 1140 / 9880 = 0.1154

B) P(none is mundane) = (17+3)C3 / 40C3 = 0.1154

C) expected value = 3 * 20/40 = 1.5

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