5.2 × 10⁹ in standard notation is <span>5,200,000,000.
If 1 billion people (1,000,000,000) didn't have access to clean drinking water, then that means as a percentage, 19.2% of the world population didn't have access to clean drinking water.
You can work this out by doing </span>
× 100 (which is 19.2<span>307...%) and then round to the nearest tenth (which is 19.2%). </span>