For four years, Marty Campbell invested $$5,000$ each year in Harley$-$Davidson. The stock was selling for $$86$ in $2017,$ $$74$ in $2018,$ $$57$ in $2019,$ and $$77$ in $2020.$

a$.$ What is Marty's total investment in Harley$-$Davidson?

b$.$ After four years, how many shares does Marty own? $($Round your intermediate calculations and final answer to nearest whole number.$)$

c$.$ What is the average cost per share of Marty's investment? $($Use the rounded number of shares computed in part b$.$ Round your answer to $2$ decimal places.$)$