The percentage of a number means the value of that number expressed as a fraction of 100. So if for example, the population of the class is 30 students, then 15 students is a portion of the entire class population and can also be expressed as a percentage. So, you start by taking note of the fact that the entire class population is 100%. Hence, the 15 student part of it is a fraction of 100% and that can be derived simply as 15 divided by 30, and the result times 100. The final answer would now be called the percentage of 15 students, which is 50%.
Let us now go back to the question at hand. The world population is given as 6.9 billion, that is, 6,900,000,000. Also the population of the U.S is given as 314 million or 314,000,000. The population of the U.S is a portion of the entire world population, and to calculate it as a percentage of the entire world (like the class example I started with), you would need to express the U.S population as a fraction of the world population and multiply the result by 100.
Hence you now have;
Percentage population = [314000000/6900000000] x 100
Percentage population = [314/6900] x 100
Percentage = 0.04550725 x 100
Percentage = 4.55072464...
Therefore, the percentage of the world population living in the U.S is 4.55%