I took the same exam today and it can easily trick you out I think. The graph was on figure 3.7 in cap 698. Outside -20, PA 18000, 4000lb and mixture leaned to 25F of peak so it came out as 1050pfm but if you used the full rich curve it came out at some less and 1370 if you used the one on the far right. So that's the only reason I can think of making it worth 3 marks.