Electric field in parallel plate capacitor

[faen]

Homework Statement

The parameters of an air-filled parallel plate capacitor are: d = 5mm and A = 500cm2. The
charge of the electrodes are Q = 10nC and −Q. Inside of the capacitor in the middle point
there is a point charge of 0, 02Q. Find the magnitude of the force acting to the point charge!

Homework Equations

I'm guessing these formulas

C=Q/V = ε0 * A/d

V = E*d

The Attempt at a Solution

The answer is supposed to be 4.52 * 10^-6 N

However I plug in the values and get 9.02 N.. Am I using the wrong formulas, or am I just calculating it wrong?

 

[rude man]

You don't want to compute capacitance.

What did you get for the E field between the plates? You can use for example σ = D where σ = surface charge density and D = εE. Or use Gauss' theorem.

Then F = EQ' where Q' = 0.02Q.

The given answer is correct BTW.