If you turn the percentage figure into a vulgar fraction (cue for gags!), add the top number (I can't remember whether that's the numerator or denominator) and the bottom number together. That gives you the new bottom number, and the top number stays the same. This will then give you the fraction that has to be deducted from the tax inclusive price to calculate the tax-less price.

Eg Tax free price 1.00

25% tax (ie 1/4) 1.25

To work it back, 1+4 = 5, top number stays the same, therefore deduct 1/5. Lo and behold 1.25 - 1/5 =1.00.

So in your problem deduct 6.5/106.5.

Clear as mud eh?