#### Answer

An equation that expresses a relationship between two or more variables, such as $H=\frac{9}{10}(220-a)$, is called a/an $\textbf{formula}$. The process of finding such equations to describe real-world phenomena is called mathematical $\textbf{modeling}$. Such equations, together with the meaning assigned to the variables, are called mathematical $\textbf{models}$.

#### Work Step by Step

The answers for this exercise are described by section P:1 of this textbook. By reading the part that talks about formulas and mathematical models, it can be seen that the blanks are to be filled with $\textbf{formula, modeling}$ and $\textbf{models}$, respectively.