# Math Help - Probability

1. ## Probability

Let X denote the numebr of flaws along an 100m reel of magnetic
tape. If in average there are 25 flaws every 100m of tape find the
probability that a random 200m tape contains more than 30 flaws.

2. Originally Posted by shogunhd
Let X denote the numebr of flaws along an 100m reel of magnetic
tape. If in average there are 25 flaws every 100m of tape find the
probability that a random 200m tape contains more than 30 flaws.
Use the Poisson distribution.

3. Originally Posted by mr fantastic
Use the Poisson distribution.
Thank you. So my lambda in this case is 25 and the probability of being more than 30 = 1 - Pr(x<=30) in the Poisson distribution.

4. Originally Posted by shogunhd
Thank you. So my lambda in this case is 25 and the probability of being more than 30 = 1 - Pr(x<=30) in the Poisson distribution.
X is the random variable number of flaws in 100 m of tape.

X ~ Poisson $(\mu_X = 25)$.

But you want Pr(Y > 30) where Y = 2X. And $\mu_Y = 2 (25) = 50$ .....