seems easy; though, when I attempt to write out the solution, it seems that I'm missing a step? Could you help explain how the solution is calculated?...

The answer to this seems easy; though, when I attempt to write out the solution, it seems that I'm missing a step? Could you help explain how the solution is calculated?

In a single-period inventory situation, the probabilities that demand will be 1, 2, 3, or 4 units are .1, .4, .3, and .2, respectively. If 3 units are stocked, what is the probability of selling all of them?