Answer
$$2\frac{1}{2} \ billion $$
Work Step by Step
We subtract the new population from the original population to find the increase. As an estimate, the increase is:
$$ 4 \ billion - 1 \ billion \approx 3 \ billion$$
The exact increase is:
$$ 3\frac{3}{5} \ billion - 1 \frac{1}{10} \ billion \\ 2 + (\frac{6}{10} -\frac{1}{10}) \ billion \\ 2+ \frac{5}{10} \ billion \\ 2\frac{1}{2} \ billion $$
