Solution
= Solution
Conservation of the parcel's <entropy> makes $p/\rho^\gamma$ constant along its trajectory. Its initial value is $p_0/\rho_0^\gamma$, so
$$
\boxed{p=p_0\left(\frac\rho{\rho_0}\right)^\gamma.}
$$
This is a relation following the specified parcel; parcels with different initial <entropy> can have different constants.