Double the number, then reduce by one-tenth the doubled number, then add 32
Example: To convert 25 C double it to 50, reduce by 5 to get 45, then add 32 to get 77 F.
The reason this works is because 9/5 = 10/5 - 1/5. And the nice thing about this is that it is an exact conversion not an estimation.
every 10 degrees Celcius is 18 Fahrenheit.
0c is 32
10 c is 32 + 18 or 50f
20c is 50 + 18 or 68f
30c is 68 + 18 or 86f
40c is 86 + 18 or 104f
Congrats you can now dress comfortably when you travel.