= Solution
For the fully water-filled reservoir, the approximate <densities> and <wave speeds> give
$$
Z_{p2}\simeq3.06\times10^6,\quad Z_{s2}\simeq1.05\times10^6\quad\mathrm{kg\,m^{-2}\,s^{-1}},
$$
and hence
$$
\boxed{R_p\simeq\frac{3.0625-2.25}{3.0625+2.25}\simeq+0.15,\qquad R_s\simeq\frac{1.05-0.900}{1.05+0.900}\simeq+0.08.}
$$
Both are positive: the reservoir has larger <P wave> and <shear-wave impedances> than the cap rock, and neither reflection reverses the chosen stress-wave <seismic reflection polarity>.
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