In a year, almost a billion people on the planet didn't have access to clean drinking water. if there were 5.2 *10^9 people in the world, then approximately what percentage of them lived without clean water

5.2 × 10⁹ in standard notation is 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  × 100 (which is 19.2307...%) and then round to the nearest tenth (which is 19.2%). 

Figure it out your self