Hands down the best mnemonic for Celsius-Fahrenheit conversions:

Ten is fifty minus forty.

That's it.
• Ten is fifty: 10°C = 50°F
• Minus forty: –40°C = –40°F
• Math checks out: 10=50–40
• Obtain any other values by deducing: since increasing from –40°C up to 10°C (+50°C) is equal to increasing from –40°F up to 50°F (+90°F), every ±5°C change means ±9°F change.

So if you ever need to deal with #Celsius or #Fahrenheit, just remember: ten is fifty minus forty.

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