split_costs.py

from typing import Union

number = Union[float, int]

def amt_owed_for_costs(costs: list[number], 
                       total: number, 
                       tax: number, 
                       tip: number, 
                       fixed_costs: list[number], 
                       num_ppl: int) -> number:
  '''
  ex:
    costs = [10, 12, 18]  
    total = 150  # before tip
    fixed_costs = [8, 10, 10]  # shared costs 
    num_ppl = 3
    tax = 13.31
    tip = 35
  '''
  base = sum(costs) + ( sum(fixed_costs)/num_ppl ) 
  return base + ( (base/total) * (tax + tip) )