C to F.

Had a sort of argument with my youngest grandson the other day about converting Centigrade (or Celsius if you insist - see post dated 3/2/06) temperatures into Fahrenheit.  He had been given the "double it and add 30" formula, which is reasonably accurate for temperatures in the mid-teens, but becomes progressively less accurate the higher or lower you go.  Here's how to do it properly.  Double the temperature in C, deduct a rounded 10% and add 32.  So for example - 23C doubled is 46, ten percent of that is 4.6 so round it up to 5 (.5 and above, round up, .4 and below round down).  Take 5 off 46 which gives you 41, and add 32 to that giving you 73 which is the correct Fahrenheit equivalent.  Using the "double it and add 30" method would give you 76 which is way too high.