How to set it up and how to do it and what the anweser is

First subtract 64.5 from 70.31
70.31-64.5=5.81
Then you multiply different numbers starting from 8.5% which is sometimes the usual tax. When you multiply 64.5 by 0.085 you get 5.48 which is slightly smaller than the number you got from subtracing the taxed amount and the before tax amount. When you get to 9% which is 0.09 you multiply that number by 64.5 which gets you 5.805 which rounds to 5.81 so the answer is 9%

The rule of rounding is 4 or below rounds down and 5 or more rounds up. The second eight is what you need to focus on. Eight is greater than 5 so you would round up which gives you the answer of 9.