I agree with John's answer, but I arrived at it using a slightly
different line of reasoning....
The 100 nC charge will spread equally over both surfaces of one
plate, while the -10 nC charge will spread equally over both surfaces of
the other plate. Thus the surface charge density on one plate will be 50
C/m^2 and -5C/m^2 on the other. Using superposition, the E field between
the plates is 55 C/m^2/epsilon0, ignoring edge effects. Integrating this
field from one plate to the other gives the potential difference (62.1 V).
Mark
Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866