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Pretend it is 100: a mental math trick

When to use it: A percent question gives no amounts at all.

Worked example

Starting from 100Bars from a start of 100: up 10% to 110, then down 10% to 99. The end is 1 below 100: −1%.9010011010011099up 10%down 10%start−1%
  1. Start with 100, or $100.
  2. Apply each change in order.
  3. Read the answer off your numbers: the gap from 100 is the percent change. A margin is profit ÷ price.

example A price goes up 10%, then down 10%. What is the overall change, in percent?

  1. 100 up 10% is 110.
  2. 110 down 10% is 99.
  3. 99 is 1 below 100: a 1% drop.

Answer: −1%

Try three

  1. A price goes up 20%, then down 25%. What is the overall change, in percent?

    Show the answer

    −10%

  2. A price goes down 20%, then up 10%. What is the overall change, in percent?

    Show the answer

    −12%

  3. A price goes down 20%, then up 25%. What is the overall change, in percent?

    Show the answer

    0%

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Used in: Percentages in Business · Markup Is Not Margin · Money & prices

More mental math tricks

This technique and its worked example are Doing Math’s own lesson text, checked by a blind re-solve and a review on September 27, 2026.