Hey guys out there who are good with numbers, i was wondering if i order say 3 henkei tf's, say starsream and a ultra magnus add on kit. how do i work out what each person pays

person a 1 henkei starscream
person b 1 henkei starscream
person c 1 henkei starscream + 1 um add on kit

obviousliy if they were all the same then i can just divide it by 3 but the 3rd person has a smaller item i want to know how to work it out fairly thanks guys. Hope that makes sense.