Size of a charge

Homework Statement

Hi
I need clarification on this problem. Calculate
a) the force between 2 charges of +1.4nC and +1.6nC on point conductors 40cm apart in air.
b)What size of charge on a third point conductor placed midway between the first two conductors would result in doubling of the magnitude of the force on the 1.4nC charge?

Homework Equations

Coulombs law= (Kq1q2)/r^2

The Attempt at a Solution

I obtained 1.25 x 10^7N for the first part. For the second part, I stated the new charge as q and the new distance as 20cm. Making the new charge the subject of my formula, i put in the new details. But my result is not correct.

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I obtained 1.25 x 10^7N for the first part.
Shouldn't it be 10^-7 ???

and cant tell you the solution

if you can post your solution, i can tell you what you did wrong

Shouldn't it be 10^-7 ???

and cant tell you the solution

if you can post your solution, i can tell you what you did wrong
Yeah! Thanks. Its -7