Celsius or centigrade is a measure of temperature where 0 degrees is the freezing point of (pure) water and 100 degrees is its boiling point. Fahrenheit has the freezing point at 32 degrees and the boiling point at 212 degrees. So the difference between 212 and 32 is 180, meaning that 180 degrees F has to fit into the same temperature range as 100 degrees C. That's where 180/100=9/5 comes from. To convert between the two temperature scales we have to take degrees F and subtract 32 first then multiply by 5/9 to get C; to convert C to F we multiply C by 9/5 first then add 32. So 14 degrees F is below freezing, and when we subtract 32 we get -18. Multiply by 5/9 and we get -10 degrees C. The temperature -40F is -40C. That's the only temperature where the numbers match.