(ps..... I would be first to admit I could very well be missing something obvious, as my head is buzzing with other issues at moment - I am only continuing with this query re the 61 as it has bugged me since I became aware of it and my curiosity is getting the better of me , so thanks again for all who have given input thus far ....and yes, I know, maybe I shouldn't be so curious and just go and have some 's and get on with issues that really need my attention).
butterflymum, if the English and Maths scores are identical for any given randomly selected individual, then yes the standard deviation would double. If the two scores vary for a given individual, the standard deviation would increase by less than double, in which case obviously a score of 237 is even beyond the 89.13th percentile.
See, if the combined percentile was much higher than the individuals, then I could understand that, with the logic of the probability of having a child who was both excellent at maths and english being lower than one or the other
eg. 1/3 good at english, 1/3 good at maths, 2/3 good at english or maths, but only 1/9 being good at english and maths.
But for the percentile to be much lower....doesn't add up.