Twenty percent of what?
Twenty percent of your money and twenty percent of mine are different amounts of money. A percent is always a share of whatever it is standing next to. Put one on, take the same one off, and see where you end up.
100, up then down 20% 96
- Against where you started
- −4
What happens to it
Drag the end of the top bar to change what you start with, and the end of the one below it to change the step.
Ready to try some? Percent increase and decrease worksheets
Where it usually goes wrong
A percent is treated as a fixed amount rather than a share of whatever it is standing beside. That is what makes 20% off and then 20% on feel like it should bring you home. It does not: the second 20% is a share of a smaller number, so £100 comes back as £96, and the missing £4 is not a rounding error.
Try this
- Press Up then down with 100 and 20%. It lands on 96, four short.
- Now press Down then up. Also 96 — the order does not rescue it.
- Drag the starting bar to a different length and watch the same 20% become a different amount of the same picture.
Where this goes next
Sale prices, interest, tax and every statistic quoted as a percentage change work exactly this way, and the missing four pounds is where a great deal of money lives.