Answer
$y=\dfrac{ks}{t}$
Work Step by Step
RECALL:
(i)
If the quantities (or variables) $x$ and $y$ are related by the equation $y=kx$, then $y$ is directly proportional to $x$, where $k$ is the constant of proportionality.
(ii)
If the quantities (or variables) $x$ and $y$ are related by the equation $y=\dfrac{k}{x}$, then $y$ is inversely proportional to $x$, where $k$ is the constant of proportionality.
$y$ is given to be directly proportional to $s$ and inversely proportional to $t$ therefore the equation that expresses the given statement is:
$y = k \cdot \dfrac{s}{t}$ where k = constant of proportionality