Format for writing reports, board papers and briefing notes?

Don’t worry too much about the difference between them. The important thing is to start with ‘To’ whoever wanted it, have a heading, have a short introduction (what it is you are reporting on), the address the main issues, and finish with a conclusion.
Try and keep your arithmetic separate and head the workings up as appendix.
If you look at answers to past exam questions you will see lots of examples (almost always one of the two compulsory questions in the past required some sort of report).