This is just a quick math article find base fare when all you have is the total.

In this example we have a Total Fare of $100, and we know the gratuity is 15%.  I need to know what the Base Fare is.   Most people will say, oh thats easy its $85.  But that is not true  $85 + 15% = $97.75

So, the easy way to do this is this:

{Total Fare}/1.{Percentage} or 100/1.15 or 86.956521

86.96+15%=100.004