How to convert Celsius and Fahrenheit
To go from Celsius to Fahrenheit: multiply by 9, divide by 5, then add 32. To go from Fahrenheit to Celsius: subtract 32, multiply by 5, then divide by 9.
A rough mental version is to double the Celsius and add 30 (so 20°C is roughly 70°F) which is close enough for everyday weather.
Useful reference points
Water freezes: 0°C = 32°F
Comfortable room: 20°C = 68°F
Body temperature: 37°C = 98.6°F
Hot summer day: 30°C = 86°F
Water boils: 100°C = 212°F
The two scales meet: -40°C = -40°F