Gordon Giles's method of converting degrees Celsius (Centigrade in my day) to Fahrenheit, Simple, really (Oxford Mail, June 16) should be ignored, especially if you are planning a career in engineering.

His example - 20F x 2, + 30 = 70 - is two degrees out.

Most people remember that -40C = -40C but, by using Mr Giles's theory, -40C x 2, + 30 = -50F.

A 10F error overstretches most people's description of the word approximate.

So, boys and girls, say after me - 20 x 9/5, +32 = 68F and -40C x 9/5, + 32 = -40F.

Accurate and simple enough - or is it time to return to school?

Michael Heavey, Oxford Road, Old Marston, Oxford