Question 88
All edges of a tetrahedron $ABCD$ are equal. Let $M$ be the midpoint of $DB$, $N$ be the point on the extension of $AB$ such that $2NA = NB$ and $P$ be a point on the altitude of $\triangle BCD$ through $D$. Find $\angle MPD$ if the intersection of the tetrahedron and the plane $(NMP)$ is a trapezoid.