ok fixing up it all up, i get this (first time i use latex!

):

$\displaystyle \frac{dF}{dt} = \frac{dF}{dr} \times \frac{dr}{dt}$

$\displaystyle F=kr^4$

$\displaystyle F=480r^4$ (from (0.5,30))

$\displaystyle \frac{dF}{dr} = 1920r^3$

$\displaystyle \frac{dr}{dt} = \frac{dF}{dt} \times \frac{dr}{dF} = 0.24 \times \frac{1}{1920(0.5)^3} = 0.24 \times \frac{1}{120} = 28.8 mms^{-1}$

hope that looks ok.

for second part how would i find t when r=1mm? i dont have any equations with t in it. (im guessing r=28.8t ?)