When charges qa, qb, and qc are placed respectively at the corners a, b, and c of a right triangle, the potential at the midpoint of the hypotenuse is 24 V. When the charge qa is removed, the potential at the midpoint becomes 17 V. When, instead, the charge qb is removed (qa and qc both in place), the potential at the midpoint becomes 13 V. What is the potential at the midpoint if both charges qa and qc are removed?