Three questions people actually ask about percentages, each with its own answer line.
Each mode is one line of arithmetic, and the working is printed under the answer so you can check it or reuse it elsewhere. Percentage change divides by the magnitude of the starting value, which keeps the sign meaningful when you begin from a negative number — a common source of wrong answers in spreadsheets.
If a rate moves from 20% to 25%, that is a rise of five percentage points but a 25% increase. Reports mix the two constantly, and the difference is usually the whole story.
Use the first mode to find the discount, then subtract it. For 15% off 240, the discount is 36 and you pay 204. Alternatively calculate 85% of 240 directly.
Because percentage change divides by the starting value. Growing from nothing is an infinite increase, so the honest answer is that the figure is undefined rather than a very large number.
No. 100 rises to 120, then falls by 24 to 96. Each percentage applies to a different base, which is why sequential percentage changes never simply cancel.