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Math4 min read

Percent heart-counting technique: A calculator can also estimate a ten-tenth.

Take the percentages down to 10, 1 and half, and most of the discount increases can be calculated on the spot, saving time for cell phones.

"Twenty-fifty percent." "Fifty-five percent increase." You don't have to pull the calculator. The core idea is to break any percentage into a combination of 10%, 5%, 1%, and add up. For example, 240 for 15%: 10% first is 24 and 5% is 12 plus 36. After a few seconds of proficientness, street shopping, tips, discounts are particularly useful, and there is no need to be influenced by the merchant's "old price" rhetoric.

Four common dismantlings.

  • 10% = original number 10, fastest and most common, first count. Bottom
  • 1% = original ÷ 100 for fine fine fine-tuning, e.g. 1% of 240 is 2.4
  • 5% = half of 10%, readily available, suitable for tips
  • 25% = original ÷ 4,50% = original = 2, integer priority

For example, 18 per cent of 240: 10 per cent is 24, 5 per cent is 12, 1 per cent is 2.4, plus 24+12+2.4 = 38.4. Whether or not to make a profit can also combine [the percentage increase or decrease] (/percentage-increase-decrease) to see the real increase or fall, e.g., the amount of loss after the first rise or fall. This approach is also used to estimate the size of the purchase price against [unit-conversion-pitfalls], which is not prone to error.

Ten per cent before fine-tuning, much faster than a direct hard-calculation of an arbitrary percentage, and sufficient to cope with the vast majority of life scenarios, with precise calculations reserved for real needs.

Validation and pit protection

The `reverse test' confirms: if about 18% of 240 is calculated, then about one sixth of 240 is weak, corresponding to the 18% scale, indicating that the results are credible. With regard to scale scaling, the question of statistical precision is referred to [rounded to valid figures] (/rounding-significant-figures), so that the approximation of the mind is not written as a precise value, but is used as a quick judgement rather than as a substitute for precision. The second half price of the supermarket is equivalent to a 70-50 discount instead of a five-50 discount, with 10 per cent and 5 per cent broken down; the real discount of 300-50% for members is about 83-33%, and the calculation is not easily monogamized.