The degree Fahrenheit (°F) is a unit of temperature named for the german physicist Gabriel Fahrenheit (1686 - 1736). In the Fahrenheit scale of temperature the freezing point of water is 32 degrees and the boiling point is 212 degrees placing the boiling and melting points of water 180 degrees apart. Zero degrees Fahrenheit indicates the lowest temperature Fahrenheit could obtain by a mixture of ice and salt.
The degree Celsius (°C) is a unit of temperature named for the Swedish astronomer Anders Celsius (1701-1744) who first proposed it. The Celsius temperature scale was designed so that the freezing point of water is 0 degrees and the boiling point is 100 degrees at standard atmospheric pressure. Since there are one hundred steps between these two reference points the original term for this system was Centigrade (100 parts).
Replace Fahrenheit with 68 and solve for Celsius:
|C = (68 - 32) * 5/9||Replace F by 68 in C = (F - 32) * 5/9|
|C = 36 * 5/9||Simplify|
|C = 20|
|20 °C = 68 °F|
Replace both temperatures with "T" in one of the equations above and solve for T:
|T = T * 9/5 + 32||Subtract 32 from both sides|
|T - 32 = T * 9/5||Subtract T from both sides (Note that T * 9/5 is the same as T + T * 4/5)|
|-32 = T * 4/5||Multiply both sides by 5|
|-160 = T * 4||Divide both sides by 4|
|-40 = T|
|-40 °C = -40 °F|
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