2016 AMC 10A Problem 8

We know that Fox has $0$ coins at the end. Then before paying the final toll, Fox had $40$ coins. Then he had $40 \div 2 = 20$ coins before the doubling. Then before paying the toll for the second crossing, he had $20 + 40 = 60$ coins. Before the doubling on the second crossing, he had $60 \div 2 = 30$ coins. On the first crossing before the toll, Fox had $30 + 40 = 70$ coins. Finally, before the first doubling, Fox had $70 \div 2 = 35$ coins. Thus, the correct answer is C .