A level maths sequences help

Hi, for part C of this question, their method makes sense, it's a geometric expression, so you'd use the Sn formula. The only thing i don't get is why you can't use this equation from GCSE

Future value = Present value x (mutiplier)^number of years.

it's simple interest and i don't see why using this equation gives me a different answer.

The formula gives the total (sum of) protits in the first 20 years. Its a geometric series.
The gcse formula is for one term of the geometric sequence in the geometric series. Its compound, not simple, interest.