The answer is 8. Look at the differences. You can see that the values are directly proportional to each other. I.e when one thing doubles, the other values double. By doubling 1, you get 2. So double the tax from 0.08 to 0.16 and double the cost from 1.08 to 2.16.
So to get from 0.08 to 0.64, you times 0.08 by 8. Multiplying the starting cost of the item (1.00) by 8 gives you 8.00
