About Money
Money questions are percentage questions wearing different clothes. What will this cost with tax added? How much am I actually saving in this sale? What does this loan cost over its whole life, rather than per month? Each is arithmetic anyone can do, and each has a standard way of going wrong.
The most common mistake is treating tax as reversible by subtraction. Taking 21% off a gross price of 121 gives 95.59, but the net price is 100 — you have to divide by 1.21, because the tax was calculated on the smaller number. The second most common is judging a loan by its monthly payment: stretching the same debt from five years to seven lowers the payment and can add thousands to the total.
These calculators show the whole picture rather than a single figure — net, tax and gross together on an invoice line, or payment, total interest and total repaid together on a loan. Results are shown to two decimal places, because cents matter when someone is checking your invoice.