## How to solve question 2?

Anyone have any guidance on this? Thanks --Haddada 20:46, 21 October 2010 (UTC)

• It would help if you wrote the question: somebody not in the class might be able to pitch in. -pm
• Here is part of my solution. For question a, suppose N is the rv. that equals to the number of balls you need and p is the prob. that a ball is put into ith bag
Then p=1/n
$P(N=k)=(1-p)^kp=(1-\frac{1}{n})^k\frac{1}{n}$
\begin{align} E[N]=\sum_{k=0}^{\infty}kP(N=k)&=\sum_{k=0}^{\infty}k(1-\frac{1}{n})^k\frac{1}{n} \\ &=(1-\frac{1}{n})\frac{1}{n}+2(1-\frac{1}{n})^2\frac{1}{n}+3(1-\frac{1}{n})^3\frac{1}{n}+... \\ &=(1-\frac{1}{n})\frac{1}{n}+(1-\frac{1}{n})^2\frac{1}{n}+(1-\frac{1}{n})^3\frac{1}{n}+... \\ &+(1-\frac{1}{n})^2\frac{1}{n}+(1-\frac{1}{n})^3\frac{1}{n}+... \\ &+(1-\frac{1}{n})^3\frac{1}{n}+... \\ &=(1-\frac{1}{n})+(1-\frac{1}{n})^2+(1-\frac{1}{n})^3+... \\ &=n-1 \end{align}
For question b, I was stucked.
--Zhao 23:50, 21 October 2010 (UTC)