Reading a risk claim

You will write three functions that turn a news claim about risk into plain numbers: the risk per 1000 people, the relative change and the number of extra cases.

The idea. A risk of 6 cases among 2000 people is 3 per 1000. A change from 3 to 4 per 1000 is a relative rise of $\dfrac{4 - 3}{3} \times 100 \approx 33.3\%$, but an absolute rise of only 1 per 1000.

Your tasks.

1. Finish per_1000(cases, people). Return the risk as cases per 1000 people.
2. Finish relative_change(old, new). Return the change as a percentage of the old risk.
3. Finish extra_cases(old, new, population). The risks are per 1000. Return the number of extra cases in the whole population.