Adding formula's to a PDF document

[graemeg48019045]

I am a very new user to this software so please tolerate my lack of knowledge

I have prepared a document ( originally in excel and then word) that it is planned to be used by an organisation by its members to record their attendance at functions.

I would like it to calculate the monies payable for their attendance i.e. if 3 members attend an $80 function it will automatically calculate $240. It will then add any donated monies to give a grand total

Below is a screen dump of the pdf page

Any help will be very much apprteciated