Statistics question (poisson distribution, multivariate )

[XodoX]

Homework Statement

On average, 2.5 telephone calls per minute are received at a corporation's switchboard. Making appropriate assumptions about the distribution ( provide justification ), find the probability that at any given minute there will be more than 2 calls.

Homework Equations

No idea.

The Attempt at a Solution

No idea.

Homework Statement

Let the joint p.d.f of X and Y, f(x,y), be given by

.....x
...1...2...3
y..1...0.3..0.2..0.1
...2 ...0.1..0.1..0.2
(ignore the dots)
a) Determine marginal densities
b) compute the means and variances of X and Y
c) Calculate sigma(?) xy = cov (X,Y) and the correlation coefficient ρ. Are X and Y independent ?
d) Let Z = X+Y. determine k(z)+P(Z=z), z = 2,3,4,5. determine the mean and variance.

Homework Equations

good question

The Attempt at a Solution

I don't have one

Well, how do I do this? I do not understand it. If anybody has some insight on either problem, I'd appreciate it.

 

[XodoX]

Got the first one now. About the 2nd one - I THINK I now have b and partly c. but c required the marginal densities, correct ( for the correlation coefficient ) ? I don't know how to do that. The example in the book just tells me ok here it is..without explaining how to get it.