Mental math

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:

1. Double the given temperature in Celsius.
2. Move the decimal over to the left one and subtract that from the doubled number.
3. 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

1. 37 / 2 = 18.5
2. 18.5 * 10 = 185

Multiplying by 25

First divide by 4, then multiply by 100.

Example: 89 * 25

1. 89 / 4 = 22.25
2. 22.25 * 100 = 2225

Misc

• x % of y = y % of x
• 6 weeks in exactly 10! seconds.