Answer:
Following are the solution to this question:
Explanation:
C'(t) was its time, in years, or t shift of cash per unit. That C'(1980) meaning, therefore, becomes the change of cashier's population to t = 1980.
They could either use t = 1970 or t = 1990 to approximate the C'(1980) price. When using t = 1970:
![C'(1980) = ([C(1980) - C(1970)])/([1980 - 1970])\\\\](https://img.qammunity.org/2021/formulas/mathematics/college/b7tvw38bv77jwgzkvj8ko7hew95bs4mqim.png)

t = 1990:
![C'(1980) = ([C(1980) - C(1990)])/([1980 - 1990])](https://img.qammunity.org/2021/formulas/mathematics/college/o0zh1we2ztxfcgawisxu0v9iu8gvzh0pdi.png)
