The answer you will get will not be an exact one, but it will give you an idea of the temperature you are looking at.
Fahrenheit to Celsius:
Take 30 away from the Fahrenheit, and then divide the answer by two. This is your answer in Celsius.
Example:
74 Fahrenheit - 30 = 44. Then divide by two, 22 Celsius.
So 74 Fahrenheit = 22 Celsius.
Celsius to Fahrenheit just do the reverse:
Double it, and then add 30.
30 Celsius double it, is 60, then add 30 is 90
30 Celsius = 90 Fahrenheit
Remember, the answer is not exact but it gives you a rough idea. http://www.memorymentor.com/