[![enter image description here][1]][1]

The dotted lines represent equipotential and the solid lines represent electric field.
potential at a = -50V
potential at b = -20V
The question is: What is the electric potential at c, the middle point between a and b?
And the answer provided is greater than -35V.

Can someone please explain to me why this is so, qualitatively and quantitatively?


  [1]: https://i.sstatic.net/XzjWx.jpg