Tax

Package: FINANCE_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

Tax questions come in three shapes: how much tax is due on an amount, what the total is once tax is added, and how much tax is already inside a tax-inclusive price. Solve has a phrase for each, so you do not have to remember which way to multiply.

tax on 100 at 20% // 20
100 + 20% // 120
tax off 120 at 20% // 100
tax in 120 at 20% // 20

tax on gives the tax, not the bill. The bill is 100 + 20%, which reads as a twenty percent increase. Going the other way, tax off takes a tax-inclusive total back to the pre-tax amount and tax in (also tax of, tax from) pulls out the tax already inside it. vat is accepted everywhere tax is.

On money the tax is exact, rounding the half-cent the same way the rest of the currency arithmetic does rather than the way a drifted double would, and that holds whether the tax is added, taken off, or pulled out.

tax on $0.10 at 15% // $0.02
tax off $0.09 at 20% // $0.08
tax in $0.09 at 20% // $0.02

No tax rate is ever assumed. You state it, because the correct rate depends on where you are and what you are buying.