Not only are there compressibility questions in the exam, but the answers to all the flight computer questions are all calculated on a CRP-5. Since others may come up with slightly different answers and the CAA has admitted that the answers are too close together for some CRP-5 questions you may get the wrong answers with your ARC-1. It is worth around £75 to buy a CRP-5. The alternative may be an extra two months and a £50 resit fee!
Also the CRP-5 is an excellent computer, with many useful notes printed on it reminding you of how to solve the problem - for example the instructions of when and how to apply compressibility! So much that it is almost cheating to use one in the exam
Best of luck.
Send Clowns
[Gen Nav instructor, BCFT]