F1-F9 (F5)

For some people it is the toughest (usually because they are not good at calculations and/or because it is their first ACCA exam). But other people find others of the F6 to F9 exams the toughest. F1 to F4 are very much just exams involving learning rules. F5 to F9 require some learning of rules, but much more understanding of what is happening. As to which is the toughest, it depends very much on how much you understand and how good you are at arithmetic. The Paper F5 exam is 50% applying rules in the calculations, and 50% proving that you understand what you are doing. However, just as for all the exams after F4, it means studying hard and practicing a lot.