# Math : Binomial + Poisson

1. Feb 14, 2016

### CryptoMath

1. The problem statement, all variables and given/known data
I need to find the variance of Y.

The number of errors on a page follows a Poisson distribution with lambda = 0.40 average . Y = the number of pages without error among the first 112 pages .

2. Relevant equations

3. The attempt at a solution
In Poisson, I know that Variance = lambda= 0.40
In Binomial, Variance = n*p*q
1-p = q
Do I simply do 112 x 0.4 x 0.6 to find the variance of Y? How am I supposed to do it?

2. Feb 14, 2016

### andrewkirk

First use the Poisson to work out the probability of a page having no errors. Then use that number in the Binomial distribution to solve the problem.

3. Feb 14, 2016

### CryptoMath

I got this resolved by doing

poissPdf(0.4,0) =p
n = 112

variance with binomial = n*p*q