How 2 calculate subtotal amount?

Hey guys!

I need help with this math:

if I have Subtotal amount = 10
Tax rate 8.5%
then Grand Total = 10 + (10 * 8.5 / 100) = 10.85

Now what if I have Grand total =10.85 and tax rate 8.5% - how to calculate the Subtotal ???

Thanks

Multiply your total by 1.0825.

are you sure this right? because 10.85 * 1.0825 = 11.745125 (not = 10) - how you got the number 1.0825 anyway?

Calculating the total including 8.5% tax can be done by multiplying the subtotal by 1.085:
10.00 * 1.085 = 10.85

Calculating the subtotal from the total including the 8.5% tax is done by apply the mathematical opposite (division):
10.85 / 1.085 = 10.00

thanks