Probability 80858

During the exam, the student takes 3 questions out of 30. He is ready for 20 of them. Find the probability that he draws at most 2 that he knows.

Correct answer:

p =  0.7037

Step-by-step explanation:

r=3020=320.6667 p0=(03)r0(1r)30=(03)0.66670(10.6667)30=10.666700.33333=2710.037 p1=(13)r1(1r)31=(13)0.66671(10.6667)31=30.666710.33332=920.2222 p2=(23)r2(1r)32=(23)0.66672(10.6667)32=30.666720.33331=940.4444  p=p0+p1+p2=0.037+0.2222+0.4444=0.7037

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