Expected number of passengers for the flight One of the interview questions that I was asked was :

The small commuter plane has 30 seats. The probability that any
  particular passenger will not show up for a flight is 0.1, independent
  of other passengers.
Design a function to get largest m value such that expectedshow[m] =
  expected number of passengers showing up if 30 + m tickets are sold
  for a flight and expectedshow[m] ≤ 30.

I could not think of a way to approach this question. Can somebody explain it ?

If $n$ tickets are sold, then the expected number of passengers that will show is $0.9n$, by linearity of expectation.
However, in this problem the variable is not $n$, but $m$, where $n=30+m$.  Hence the number of expected passengers that will show is $0.9(30+m)$.  Now consider those values of $m$ that make this at most 30; choose the largest.  You should get $$\frac{30}{0.9}-30=3.\overline{3}$$
Most likely the interviewer is looking for the floor of this, namely $m=3$.
