Bitcoin & money learning · Retrospective
Calculate a change from the correct base
Check which observation belongs in the denominator.
Source foundation
CPI measures changes in consumer prices; it is not an individual household budget. BLS: CPI questions and answers.
A worked example
A fictional index rises from 120 to 126. The change is 6 divided by 120, or 5 percent. Dividing by 126 instead answers a different arithmetic question. A worksheet should show the earlier value, later value, and formula together so a reader can reproduce the result.
Original hypothetical example. Amounts, people, events, and scenarios above are invented for learning; they are not historical observations or forecasts.
What to take from it
A familiar-looking percentage can still use the wrong base.
Try it on paper
Calculate the percentage decline from 126 back to 120.
Keep the assumptions beside your answer. Check the result before applying the idea to a different situation.
Reading and limits
The linked source supports the foundation statement. The worked example and exercise are NAKAMOTO's educational analysis. This note does not establish current prices, product terms, investment suitability, or wallet compatibility.
Source reviewed 13 September 2026. For operational decisions, consult the current official documentation and the details of your own circumstances.