Bag A contains 2 white balls, bag B

contains 2 black balls. You randomly

pick a ball from bag A and place it

into bag B, then you randomly pick a

ball from bag B and place it into bag

A - this will be called an

**iteration**.

Note that after any number of

iterations the two bags will each

contain 2 balls. What is the

expected number of iterations needed

to go from the initial configuration

of 2 white balls in bag A and 2

black balls in bag B to a final

configuration of 2 black balls in

bag A and 2 white balls in bag B?