Formula to find the base amount


I have new taxation laws that apply my organization from January.

The current taxes are 10% +10%
So if it was 100 it came upto 120

Now they have introduced one more variable. It is 10% + 10% + (another 5% on base with one 10%)

so 100 + 10 + 10 +( 110*5%) =125.5

I know how to set up the formula to calculate the tax. But how do I get the base amount if I have the amount with taxes?

For example, How do I set up a formula to find 100 if I know the total amount is 125.5?