Ok well is this of CIE?
If it is then check out gradethresholds!! What is it??
Well it shows u the min. marks needed to get a particular grade on each papers of 1 subject.
Check this out (its for a level chem.)
http://www.freeexampapers.com/FreeExamPapers.com_.php?__lo=QSBMZXZlbC9DaGVtaXN0cnkvQ0lFLzIwMDggTm92Lzk3MDFfdzA4X2d0LnBkZg==just open it or save it.
CIE makes these for all the exam sessions. they vary with year, this is of O/N 2008.
Your asking the lowest grades required for each paper to get an A in that subject, right??
If so then its not quantitative to say the "grades". The marks or % gives a better idea!
I don't think CIE give % for each papers of that subject...they give % for the subject only. Otherwise u can see it from the previous pdf. easily.
Other way to find out is looking at the weightage of each paper.
In ur eg. i think weights are in highest to lowest: P4, P2, P1,P5 and P3.
that way u can balance the "grades" for each paper and find the overall grade in that sub.
hope its clear