What is the formula for calculating the inclusive tax amount from a given amount?

I know its an old question, but some how it may useful for others.

Suppose the item price is 20.00 (inclusive of tax)

Tax rate is 14.5
 

we need to calculate exclusive price the formula is as follows.

tax excluded price  = item price / ((tax rate /100) + 1 )
 

Eg: 20 / ((14.5/100) + 1)) = 17.4672489.

The detailed documentation can be found here for calculating tax inclusive and exclusive in billing applications