I have one white ball, one yellow ball, one red ball, one black ball. I put the four balls in a nontransparent box. I pick a ball from the box to see its color and put it back to the box.
Assuming picking is random, how many times on average do I need to pick in order to see all four colors?
If I'm lucky, I only need to pick four times. If I'm out of luck, I may get red, red, red, red, red, red,....
But what is it on average?