Celsius to Fahrenheit
To convert Celsius to Fahrenheit, multiply by 1.8 and add 32. Water freezes at 0 °C or 32 °F and boils at 100 °C or 212 °F. Normal body temperature, 37 °C, is 98.6 °F. The two scales read the same at −40, which is the only temperature where no conversion is needed.
How to convert °C to °F
Temperature is the one everyday conversion that is not a simple multiplication, because the two scales do not share a zero. That is also why you cannot convert a temperature difference the same way: a rise of 10 °C is a rise of 18 °F, not 50 °F. Multiply differences by 1.8 and leave the 32 alone.
Questions
Double the Celsius and add 30. Twenty becomes 70 against a true 68, and 30 becomes 90 against 86. Close enough for weather, wrong for an oven.
Because it is a conversion of the round figure 37 °C, taken from a nineteenth-century German study. Modern averages sit slightly lower, nearer 36.6 °C.
For practical purposes yes. The name was changed to Celsius in 1948 to avoid clashing with the centigrade angular unit used in surveying.
At −40. Minus forty Celsius is minus forty Fahrenheit, the single crossing point of the two lines.
Kelvin shares Celsius degree size but starts at absolute zero, so K = °C + 273.15. Nothing in weather or cooking needs it.