comp_sci_kid

That is just their recommended method for solving this type of problem. That is a very basic example so you could just take the(10,000 * 1) and add (2,000 * 0.5) and get the same thing.
It’s better to stick with their method because when they make the question more complicated (splits, stock dividends, different things having different amount of months outstanding) it will be easier to solve. See the next example on page 65.
Good Luck