Carbon dioxide comprises, on average, about 397 parts per million of

397 --> \(10^{6}\)
x --> \(4*10^{9}\)

x = \(\frac{397*(4*10^{9})}{10^{6}}\) ≈ \(\frac{400*4*10^{9}}{10^{6}}\)=\(1600*10^{3}\)=1 600 000

The answer is C

Given:
1. Carbon dioxide comprises, on average, about 397 parts per million of the Earth's atmosphere, on a molecular basis.
2. In an even sample of the Earth's atmosphere consisting of 4 billion (4000 millions) molecules
3. 1 billion = 1,000,000,000

Asked: Approximately how many of those molecules would be expected to be carbon dioxide?

1. Carbon dioxide comprises, on average, about 397 parts per million of the Earth's atmosphere, on a molecular basis.
2. In an even sample of the Earth's atmosphere consisting of 4 billion (4000 millions) molecules

Approximately number of those molecules expected to be carbon dioxide = 397 parts/million * 4000 millions = 397*4000 = 1,588,000 = 1,600,000 approx

IMO C

We can create a proportion (using 1 for 1 million and 4,000 for 4 billion since 4,000 million is 4 billion):

397/1 = x/4,000

x = 397 * 4,000 ≈ 400 * 4,000 = 1,600,000

Answer: C