This section is from the book "Eating To Live", by John Janvier Black. Also available from Amazon: Eating to Live.
To reduce Centigrade reading to Fahrenheit take 3/5 of number of degrees Centigrade and add 32°.
To reduce Fahrenheit reading to Centigrade, subtract 32° from number of degrees Fahrenheit and take 5/8 of the remainder.
The reason of the fraction is readily seen when we remember that 100° Centigrade corresponds to 2120 - 320 or 1800 Fahrenheit
Another way from Centigrade to Fahrenheit: Multiply number of degrees Centigrade by 1.8 and add the result to 320. For example: 15 degrees Cent: X 1.8=27.0. Add this to 320, and we get 590 Fahrenheit
 
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