From the expression 2f + 5(8-f), we know that Karla caught f purple dot fish, and (8-f) green dot fish, because green dot fish gives 5 points.
2f -points for the purple dot fish, f is a number of the purple dot fish.
5(8-f) - points for the green dot fish, (8-f) is a number of the green dot fish.
So,
altogether she made 8 attempts.
One attempt cost 50 cents,
8 attempts cost 8*50=400 cents = $ 4.
Answer is B. 4 (dollars).