Marking

You do not lose marks twice for the same mistake. If you make a mistake on one part then you obviously lose marks for that mistake, but you still get full marks for the later bits if you are doing the right thing on your figure. (Provided of course that the marker can follow your workings - this is why it is so important to set out your workings clearly because it its impossible int he exam not to make mistakes somewhere - even if it is just a silly calculator error :-) )