- #1

prime-factor

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- 0

## Homework Statement

Eighty per cent of all scheduled trains through

Roma Street station arrive on time. If 10 trains

go through the station every day, find:

**a**the probability that at least 8 trains are

on time

**b**the probability that at least 8 trains are

on time for 9 out of the next 10 days.

## Homework Equations

P(X = x) =

^{n}C

_{x}p

^{x}q

^{n − x }

## The Attempt at a Solution

P(X>=8)

n=10

x=8+9+10

p=0.8

q=0.2

P(X=8)+p(X=9)+p(X=10) = 0.6778

^{10}C

_{8}0.8

^{8}0.2

^{2}+

^{10}C

_{9}0.8

^{9}0.2

^{1}+

^{10}C

_{10}0.8

^{10}0.2

^{0}

**b**I guess I would use:

P(X=8)+p(X=9)+p(X=10) = 0.6778

But I am not sure how. Please help!