In the same chapter that has interest formulas, there are formulas for population growth and radio active decay.
Answering the question of why do we need to know half life of radio active elements is easy. Since we have to dispose of the waste from power plants, x-rays, etc we need to know how long they need to be buried but when asked about population growth that can be a much harder thing.
So why do we need to be able to predict population growth?
We need to predict growth for:
1. Figuring out how much housing is needed for the additional population.
2. Deciding the number of cars that will be traveling the roads which determines how many new roads should be built or how many roads need to be expanded.
3. How many portables will be needed to added to the local schools for the increasing population or will new schools be needed? What about colleges?
4. How much more money will be needed for the growing population in the over 65 sector?
5. How much more food needs to be grown to keep up with the increasing population?
6. How much more demand for energy will the increased population require?
If students visited local census records, they could find enough points to calculate the growth of their city or country using a spread sheet. Once the rate has been calculated, students can use that rate of growth to predict the population in 10, 20, 30, or 50 years. Using the rate of new house construction, they can project if there will be enough housing available in 10 years for the predicted population.
This would create a connection between the theoretical and the real use of calculating population growth.