Point charges and force of repulsion

[kbyws37]

Two point charges are separated by a distance r and repel each other with a force F. If their separation is reduced to 0.280 times the original value, what is the magnitude of the force of repulsion between them?


I don't really understand this question and don't know where to start.
Would I start with the equation
E = ((k)(Q)/r^2

what follows next?

 

[JeffKoch]

What is the equation for the repulsive force between two point charges, and how does it scale with r?

 

[kbyws37]

What is the equation for the repulsive force between two point charges, and how does it scale with r?

It would be Coulomb's law of

F = ((k)(q1)(q2))/(r^2)

So it would be -0.280 times the original r value?

The final answer is 12.8 F but I still don't understand.

 

[JeffKoch]

Where do you get the minus sign?

O.K., you know that F scales as 1/r^2. When r gets smaller, F must get larger. When r is half it's original value, how much larger does F get?