# 40. What does it mean if two quantities vary directly?

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- What does it mean if two quantities vary directly?
- In your own words, explain how to solve a variation problem.
- What does it mean if two quantities vary inversely?
- Explain what is meant by combined variation. Give an example with your explanation.
- Explain what is meant by joint variation. Give an example with your explanation.
- We have seen that the daily number of phone calls between two cities varies jointly as their populations and inversely as the square of the distance between them. This model, used by

telecommunication companies to estimate the line capacities needed among various cities, is called the gravity model. Compare the model to Newton’s formula for gravitation on

page 400 and describe why the name gravity model is appropriate.