RO-TST 2010 Problem 16

Let $\mathcal{L}$ be a finite collection of lines in the plane in general position (no two lines in $\mathcal{L}$ are parallel and no three are concurrent). Consider the open circular discs inscribed in the triangles enclosed by each triple of lines in $\mathcal{L}$ . Determine the number of such discs intersected by no line in $\mathcal{L}$ , in terms of $|\mathcal{L}|$ .

B. Aronov et al.