Answer
$$3 \frac{1}{20} \ billion $$
Work Step by Step
We subtract the new population from the original population to find the increase. As an estimate, the increase is:
$$ 10 \ billion - 7 \ billion = 3 \ billion$$
The exact increase is:
$$ =10\frac{1}{4} - 7 \frac{1}{5} \ billion \\ 3 + (\frac{5}{20} - \frac{4}{20} ) \ billion \\ 3 \frac{1}{20} \ billion $$
