# Thread: simple probability question - need help

1. ## simple probability question - need help

Westjet had the second best on-time arrival rate of all airlines in North America, just behind Hawaiian airlines. The average for all North America airlines is 70%. In a particular 90 day rolling average the on-time arrival rate for Westjet was 80 %.
Consider a test conducted by randomly selecting 9 Westjet flights and observing whether they arrive on time.
Find the probability that at least 3 flights arrive late.

2. Originally Posted by proski117
Westjet had the second best on-time arrival rate of all airlines in North America, just behind Hawaiian airlines. The average for all North America airlines is 70%. In a particular 90 day rolling average the on-time arrival rate for Westjet was 80 %.
Consider a test conducted by randomly selecting 9 Westjet flights and observing whether they arrive on time.
Find the probability that at least 3 flights arrive late.
Binomial distribution problem. The number ontime from a sample of nine has a binomial distrinution $B(9,0.8)$, so you want:

$
p=\sum_{i=3}^9b(i;9,0.8)=1-(b(0;0,0.8)+b(1;9,0.8)=b(2;9,0.8))
$

CB