About the Percentage Calculator
Percentages are the most-used bit of school maths in adult life — and the easiest to get wrong under pressure. A discount that says “30% off, plus an extra 20% off the reduced price” is not 50% off, and percentage increases and decreases do not cancel out symmetrically.
Having the three standard formulas in one place — percent of a number, the reverse “what percent is X of Y”, and percentage change — covers almost every everyday case, from splitting a restaurant bill to checking whether your salary raise beat inflation. For shopping in particular, the discount calculator handles the sale-price, “what discount did I actually get” and original-price versions in a single step. The same weighted-average idea decides a school result, where credits rather than course count set how much each grade matters — that is what the GPA calculator does.