Mental math
Last updated: 15 May 2023 ·
Posted in wiki#notes
Converting between Celsius and Fahrenheit
For a quick rough approximation, these formulae are useful:
- C = (F - 30) / 2
- F = (C * 2) + 30
Some degrees easier to memorise:
- 0 C = 32 F
- 100 C = 212 F
- −40 C = -40 F (equal)
- 28 C = 82.4 F (digits reversed)
- 16 C = 60.8 F (~61 F)
For precise calculation:
- Double the given temperature in Celsius.
- Move the decimal over to the left one and subtract that from the doubled number.
- Add 32 to that result to get the answer.
Example: 24 C -> 48 - 4.8 -> 43.2 + 32 -> 75.2 F
Multiplying by 5
For easier calculation, first divide the number by 2, then multiply the result by 10.
Example: 37 * 5
- 37 / 2 = 18.5
- 18.5 * 10 = 185
Multiplying by 25
First divide by 4, then multiply by 100.
Example: 89 * 25
- 89 / 4 = 22.25
- 22.25 * 100 = 2225
Misc
- x % of y = y % of x
- 6 weeks in exactly 10! seconds.