Everything in the threads above is correct, however sometimes a person needs to use mental arithmetic. As a Canadian who is 63, I grew up with F. but now 'we' use C. The old codgers have had a tougher time converting.
The quick way is to take the C degrees x 2 plus 30 = F degrees.
For example 15C = (15 x 2) + 30 = 60F. The perfect answer would be 59.
To go from F degrees, subtract 30 and divide by 2 to get C.
For example 72F = (72-30)/2 = 21C. The perfect answer would be 22.
Since the purpose of these conversions is comprehension of what the temperature FEELS like, the error of one degree does not mean much IMO.
Good luck.