I have done similar questions but this one does not give a value for r. Should I give it any value?
No need...
$\displaystyle relative\ error=\frac{absolute\ error}{exact\ value\ of\ that\ being\ measured}$
$\displaystyle =\displaystyle\huge\frac{{\pi}r^2-{\pi}\left(r-\frac{r}{400}\right)^2}{{\pi}r^2}=\frac{{\pi}r^2-{\pi}r^2\left(1-\frac{1}{400}\right)^2}{{\pi}r^2}$
or
$\displaystyle \displaystyle\huge\frac{{\pi}\left(r+\frac{r}{400} \right)^2-{\pi}r^2}{{\pi}r^2}=\frac{{\pi}r^2\left(1+\frac{1} {400}\right)^2-{\pi}r^2}{{\pi}r^2}$
which may be factorised, removing $\displaystyle r$ and $\displaystyle \pi}$ completely.
Those parameters are needed for the "absolute" error,
but not for the "relative" error.