I think that the mistake that could have been made was to have used the lean mixture line on the right hand side. This is the one with the 'lean mixture' comment and the arrow pointing to it. If you used this line, then you would have arrived at an answer of 1375. You had to use the one that was in the middle alongside the 4000lb line. This would lead you to an answer of 1050. If you look at the link earlier in this post you will see what I mean.
But I do agree that it seemed relatively for 3 points (especially as compared to one of the other CAP questions that required quite a lot of working for only 2 points)