Convert Centigrade to Fahrenheit Mentally

My first Instructable.

While on holiday in the Canary Islands we were kept informed of the time and temperature by a number of public digital displays. Naturally the temperature was in Centigrade. Fine, except that in spite of the UK going metric, atmospheric temperatures do not mean much to me unless they are in Fahrenheit (it is an age thing, you know!) so I got to thinking how best to convert them. The following was my train of thought.

The formula is, of course :

F = (9/5)C + 32

The problem is the fraction 9/5 until I realised that with a little elementary algebra, this could be transformed into a much simpler form.

Consider

9/5 = 18/10 = (20-2)/10 = 2 + 0.2

Wow ... Eureka!

Step 1: Examples

So, in order to convert degrees Centigrade to Fahrenheit we have now a new formula:

F = (2C + 0.2C) + 32

... and that we can do in our head! We merely multiply the centigrade number by 2, subtract a tenth of that, and add 32.

Let's try some examples.

Firstly, something that we all know. 100C = 212F

100 x 2 = 200
1/10 = 20 (move the decimal point)
Subtract = 180
+32 = 212F

Let's try another one, 60C

60 x 2 = 120
1/10 = 12 (move the decimal point)
Subtract = 108
+32 = 140F

Or 25C

25 x 2 = 50
1/10 = 5
Subtract = 45
+32 = 77 (see below)

Step 2: Finally

If you are only interested in an answer to the nearest degree then it is OK to round the tenth part to the nearest degree if it is not a whole number, to ease the mental strain.

i.e. 19C

19 x 2 = 38
1/10 = 4 (rounding 3.8)
Subtract = 34
+32 = 66F (instead of 66.2F)

Now it is your turn. Try 30C, 62C and 1,000C.

Easy, isn't it?

You can always check by entering "XXX Centigrade in Fahrenheit" into Google.

If only it was that easy the other way round.
If you have a mental method for converting degrees F to degrees C then let's hear it.