When charges q_a, q_b, and q_c are placed respectively at the corners a, b, and c of a right triangle, the potential at the midpoint of the hypotenuse is 20.8 V. When the charge qa is removed, the potential at the midpoint becomes 11.3 V. When, instead, the charge qb is removed (q_a and q_c both in place), the potential at the midpoint becomes 16.7 V. What is the potential at the midpoint if only the charge qc is removed from the array of charges?