#### Answer

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

#### Work Step by Step

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.)