3 Robert maintains a server farm with 20 identical computers

3. Robert maintains a server farm with 20 identical computers. For convenience, these computers are labeled C,, Cn. A are labeled C1, C2,... . Cn. A software virus has infected exactly 3 machines, and every computer is equally likely to be affected by the virus.

Solution

There are 20C4 ways to choose any four computers, and 17C4 ways to choose 4 nondefective computers.

Hence,

P(all nondefective) = 17C4 / 20C4 = 0.49122807 [ANSWER]