In page 4 of exam report, there is a sentence "By between 15% and 25%, judging by the numbers given — say 20%". I don't understand how 15% and 25% come. Thanks in advance.
I wouldn't spend too long analysing the individual figures, as a wide range of assumptions was acceptable here. All they did in this case was look roughly at the adjustments they had made to bring other years on level (which themselves had flexible assumptions too), and made the assumption that the adjustment here was something along the same lines. There are a lot of past SA3 exam questions where you have to make judgements when adjusting past data to make it applicable going foward. The examiners will give credit for any reasonable approach - what's important is that you've realised that the figures do need adjusting for XYZ - the actual numerical adjustments only have to be in the right ballpark.
