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