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As frobozz, Giovanni Ciriani and David pointed out, the pouring process can be tracked using trilinear coordinates.

By the way, if the trilinear coordinate system in this problem wasn't very intuitive, here's one way to think about it.

Trilinear, trī-lin′ē-ar, adj. consisting of three lines.

Trilinear and Areal Coordinates.—Consider a fixed triangle ABC, and regard its sides as produced without limit.

Trilinear equations are, as a rule, dealt with in their homogeneous forms.