# Homework Help: Pls help check my answer for this probability question.

1. Aug 11, 2004

### denian

the lifespan of a species of plant is a random variable T ( tens of days ). the probability density function is given by

f(t) = { (1/8)e^(-t/8) , t > 0
{ 0, otherwise

(i) find the cumulative distribution function of T and sketch its graph

F(t) = { 0, t < or = 0
{ 1 - e^(-t/8) , t > 0

(ii) find the probability, to three decimal places, that a plant of that species randomly chosen has a lifespan of more than 20 days

i do it this way..

F(2) = .....
= 0.221

P(lifespan > 20 days) = 1 - 0.221 = 0.779

(iii) calculate the expected lifespan of that species of plant
i get E(t) = 8
therefore, expected lifespan of that species is 80 days.

please tell me if i am correct in the questions above. sorry, i tried to use Latex but my attemp failed.