fine for the first day:- $ 0.10

fine for the second day can be :- $ (0.10 + 0.30) or $ 0.10 * 2 => $ 0.20
as the question tells us that the fine will be the one which is the lesser of two.
thus after second day fine is: $ 0.20

fine at the end of 3rd day will be :- $(0.20 + 0.30) or $(0.20 * 2) => 0.40

fine at the end of the fourth day :- $(0.40 + 0.30) or $(0.40 * 2) i.e. $0.70 or $0.80

Thus the answer is $0.70 as the fine is always the lower amount.

Great explanation . Thanks