As some of you on the mainland finally realised following DUP conversations, as unionists call the larger island lieing to the east of Ireland, and now have now reason to believe. NI is currently part of the UK, so my statement was valid. We can readily identify NI registration numbers from other UK numbers because they usually have an Z or I and use 4 digit numbers. So 5% of the cars were from the mainland, 75% across the border and 20% ROIYes it is sectarian, the "Glorious Twelth". This Link tells all.
As for your other question, percentages aren't really mathematics, they are politician's convenient alternative to mathematics, an aid to bamboozle the gullible public.
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I was going to make comments about the glorious twelfth, but Tommie will do it with more heart, and flecc with more accuracy.