Every common percentage calculation in one place, with the formula shown.
Pick a mode using the buttons. Each mode takes two inputs and shows the result and the formula used. Use mode 1 for "what is 20% of 150", mode 2 for "30 is what percent of 200", mode 3 to find the missing whole, mode 4 for percentage change between two values, and mode 5 to add or subtract a percentage from a value.