Water and then oil (which don't mix) are poured into a U-shaped tube, open at both ends. They come t

Water and then oil (which don't mix) are poured into a
U-shaped tube, open at both ends.
They come to equilibrium as shown
in Fig. 13-49. What is the density of
the oil? [Hint: Pressures at points a Oil
and b are equal. Why?]