Algebra homework? Need help!

there are the problems i have no clue about:

"In 2006, there were 299,400,000 Americans. If this nuber represented 4.58% of the world's population, how large was the worls's population in 2006?''

"The 'Spanish Flu' pandemic of 1918-1919 was among the greatest worldwide disasters. Its is not knows exactly how many people died, but estimates range from 50 million to 100 million people. If these numbers represent 2.5% and 5% of the world's population, how large was the world's polulation at that time?''

''Oxygen accounts for approximately 23.113% of the mass of Earth's atmosphere. According to the National Center for Atmospheric Research, the toal mass of the atmosphere is 5.1480x10(on the 18th power) kg. What is the total mass of the oxygen in the atmosphere?


Thank you so much for your help!!!

PART1
299,400,000=.0458x, where x is world's pop
divide by .0458
x=6537117903.930131 or approximately 6,537,117,904 people

PART2
50,000,000=.025x where x is world pop
divide by .025
x=2,000,000,000

OR
100,000,000=.05x where x is word pop
divide by .05
x=2,000,000,000 as it should be


PART3
let x be mass of oxygen
x=.23113*5.1480X10^18
x=1.1899 X 10^18 kg