0
$\begingroup$

16 cops and 8 suspects are sitting in a row, waiting for a train.

It would be desirable that each suspect is flanked on both sides by cops.

If they sit randomly, what is the expected value of the # of suspects so flanked ?

  • 0
    @Patrick: No, they have to be flanked on both sides.2012-02-27

1 Answers 1

3

Expectations add, so you can compute an individual suspect's odds of being flanked and then multiply by 8. Each suspect has a $\frac{22}{24}$ chance of sitting somewhere other than the ends (where said suspect cannot be flanked), and then there is a $\frac{16}{23}$ chance the person to his left will be a cop, and then a $\frac{15}{22}$ chance the person to his right will be a cop (given that the person to his left is a cop). $\frac{22}{24}\cdot\frac{16}{23}\cdot\frac{15}{22}=\frac{10}{23}$ The expected number of suspects flanked by cops is $\frac{80}{23}$

  • 0
    I'm sorry if this is basic but how do you obtain 16 out of *23*? I tried to repeat the pattern for smaller numbers of cops/suspects in order to understand, yet it didn't work out.2016-03-12