Hi,

I am trying to solve a problem from the book 'Probability Theory, The logic of Science' from chapter 3 page 71. The problem goes like this:

Suppose an urn containsN= balls,N1of color 1,N2of color2, . . . ,Nkof colork.We drawmballs without replacement; what is the probabilitythat we have at least one of each color? Supposingk= 5, allNi= 10, how many do we need to draw in order to have at least a 90% probability for getting a full set?

My solution goes something like this:

For a multinomial distribution where sampling is carried out without replacement, the probability of drawing exactly r1, r2, ..., rk balls from N1, N2, ... Nk total balls is given by

To draw m balls which have atleast k distinct colors, m should be greater than k.

So probability for drawing m balls without replacement = (Probability of drawing k distinct balls) x (1 - Probability of drawing m-k distinct balls)

This reduces to

As I am studying myself, I have no means to verify. Is this approach correct?

As to second part of the problem, I took probability of atleast one of each color as 0.9 and tried to solve for m. But m occurs in binomial expression and even by trial and error I could not get a value of m which could satisfy the above equation.

Can someone help me? Hope I was clear enough.