Hey Arsey (or anyone else for that matter)..
I got the answer in your mark scheme for 4c originally, but when I went back through at the end and re-did it, I got a different answer which I kept. So if you could please advise me what is wrong with this method that'd be much appreciated..
Taking moments about B.
CW Moments:
(RD)(d)
ACW Moments:
(mg)(7d/2) + (5mg/2)(5d/2) = (7mgd/2) + (25mgd/4) = 39mgd/4.
Therefore..
dRD = 39mgd/4.
RD = 39mg/4 = 95.55m.