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%