For other Americans, 40C is 104F.
When I have to convert C to F in my head, I start with 10C = 50F, and then every additional 5C is 9F. So 40 is 30 more than 10, 30/5 is 6, 6*9 is 54, and 50 + 54 is 104. This is easier than multiplying by 1.8 and adding 32.