Loan payment
Monthly repayment, and what the borrowing actually costs.
The sums you keep looking up
Calculators
Ctrl K
Percentages
The four questions people actually ask about a percentage.
Bill split
With a service charge, and rounded so nobody is short.
Tax add or remove
Both directions, because a price is quoted either way.
Margin and markup
Two different numbers that get confused constantly.
Cost & selling price
Price up from a cost, or work back from a price you must hit.
Discount
What it comes down to, and what you keep.
Days between
Counting inclusively, and in working days too.
Savings goal
How long it takes, or how much a month it needs.