Answer
$29\frac{rocks}{min}$
Work Step by Step
We know that
$thrust=(\frac{\Delta m}{\Delta v})(v)=f_k$
This can be rearranged as:
$\frac{\Delta m}{\Delta t}=\frac{f_k}{v}$
We plug in the known values to obtain:
$\frac{\Delta m}{\Delta t}=\frac{3.4}{11}=0.31\frac{Kg}{s}$
Now we can find the number of rocks per minute as
$\frac{\Delta m}{\Delta t}=(\frac{0.31Kg}{s})(\frac{1rock}{0.65Kg})(\frac{60s}{min})=29\frac{rocks}{min}$
