Tax reverse calculation formula.

I have a product used to sell for 118 which includes value of the product is 100 + 9% of CGST tax + 9% of SGST tax which equals 100+9+9=118.

Now I want to calculate the tax from the total cost. for example, total cost is 118, i need a help for the formula to work back 100+9+9=118.

I have tried, but i am getting wrong answer.