calculating parts per million

Most community water supplies have 0.5 ppm of chlorine added for purification. What mass of chlorine must be added to 100.0 L of water to achieve this level?

i really need help with this question i dont understand how to do it because it is completely different from the example in my book!!!

The formula for parts per million is

        ppm = mass of solute / mass of solution  x  1,000,000
In this case you are told that you have a 100.0 L of water (the solution).  Since the solution is essentially water, the density of the solution is very, very close to 1.00 g/ml.
And 100.0 L is equivalent to 100,000 mL.  If we use the density, we can find the mass of the solution:
                        100,000 mL  x  1.00 g/ml = 100,000 g

Now plug what we know into the equation for parts per million:

                          .5 ppm =  x grams of solute / 100,000 g solution  x 1,000,000

Solving for x yields  x grams of solute =  .5 x 100,000/ 1,000,000
                            x grams of solute = .5 x .1 = .05 grams of solute (chlorine)

basically if you can separate the magnitude of your units by 1000000 you will have ppm