Do you go to MCC? Is this from Madeline's worksheet? I am stuck on this too!
In this case, I believe the magnitude of F given should be differentiated w/r to r ...
$\displaystyle F = \frac{k}{r^2}$
$\displaystyle \frac{dF}{dr} = -\frac{2k}{r^3}$ where $\displaystyle \frac{dF}{dr}$ is a rate of change of force magnitude in $\displaystyle \frac{N}{m}$ and can be determined from the given info.
... The problem is poorly worded, imho. The question asking "how fast" could have been made more clear.