TiNspire users can easily solve Finance and Business related problems STEP BY STEP using the Finance Made Easy at
The extensive Time Value section of the menu shows Simple Interest, Future and Present Value, Compounded Interest, Annuities, NPV , IRR and MIRR (further down)
Lets start off with a basic tax (i.e. VAT) problem where we have to add 30% to $1000 to get $1300 .
Vice versa , we can also subtract interest from a given amount (i.e. pay check) by entering a negative % , see below: