- #1

dreamspace

- 11

- 0

## Homework Statement

Suppose that I'm collecting cards, and that in a complete collection there are

*m*items.

When buying a new card, there's an equal probability that the card is any of those

*m*cards.

Let X be the number of cards I need to buy in order to get a complete collection

What is the Expectation/Ex of X? What is the Standard Deviation?

## Homework Equations

Let [itex]X = \sum^{m}_{i=1} X_{i} [/itex], where [itex] X_{i} [/itex] is the number of cards I need to buy in order to get a new type of card when I already have i - 1 different types of cards

## The Attempt at a Solution

I figure this problem would involve probability mass function, but to be honest I'm stuck as I haven't had any probability or stats in over 10 years.

Any good pointers on how to go on with this problem?