Force between point charges and third charge to double force

[aurao2003]

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.

Please any advise. Thanks

 

[cupid.callin]

I obtained 1.25 x 10^7N for the first part.
Please any advise. Thanks

Shouldn't it be 10^-7 ?

and can't tell you the solution

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

 

[aurao2003]

Shouldn't it be 10^-7 ?

and can't tell you the solution

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

Yeah! Thanks. Its -7