Chapter 1, Equations and Graphs - Section 1.10 - Modeling Variation - 1.10 Exercises - Page 164: 17

$\displaystyle R= \frac{kP^{2}t^{2}}{b^{3}}$

Since $R$ is directly (jointly) proportional to the squares of $P$ and $t$, but inversely proportional to the cube of $b$, we have: $\displaystyle R= \frac{kP^{2}t^{2}}{b^{3}}$ (where $k$ is a constant.)