Because not all airline passengers show up for their reserved seats, an airline sells 125 tickets for a flight that holds 120 passengers. The probability that a passenger does not show up is 0.10, and passengers behave independently.

What is the probability that the flight departs with empty seats?

  • $\begingroup$ What do you need to know to start with? $\endgroup$ – Joffan Feb 1 '17 at 3:07

The number of passengers showing up is a random variable $X\sim\text{Binomial}(125,0.9)$

And what you want is

$$P(X\le 119)=F_X(119)=\\=1-P(X=125)-P(X=124)-P(X=123)-P(X=122)-P(X=121)-P(X=120)$$

You may calculate it numerically with a computer program, or do a normal approximation.

  • $\begingroup$ And it is marginally easier to calculate : $1-P(120\leq X\leq 125)$ $\endgroup$ – Graham Kemp Feb 1 '17 at 3:10

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