For easier mental C -> F conversion, take (2C)-(2C/10)+32 where C = temp in c.

16c × 2 = 32

32/10 = 3.2

32 - 3.2 = 28.8

28.8+32 = 60.8, which is exactly correct.

Or if you just want a rough estimate for weather, 2C+(22 to 32) gets you close enough with easier mental math

16x2+30 = 62, just 1 degree off.

37x2+30 = 104 is like 6 degrees off, though. The further from zero you are, the smaller the number you should add.