cash flows finding purchase of NCA

You have not said what question you are referring to. It depends on what information is given in the question. If you are only given the NBV's in the SOFP then you can only do your workings on the NBV's. If you are given the cost and accumulated depreciation figures separately then you can do your workings on them.

No - if you are given them separately then in your workings you should write up each of the two accounts and find the missing figures.