To go from celsius to fahrenheit, you take the celsius temperature, multiply it by 1.8, and add 32. you could write this as the equation $f = 1.8c + 32$. find a temperature that's the same in fahrenheit and celsius.

Short answer - 40oC or - 40o F.
Comment
If you live in northern Canada, you are well aware of this oddity. It is quite often experienced in the north.
Solve
The trick is to make one of the letters the same as the other. That is
F = C

C = 1.8C + 32       Subtract 32 from both sides.
C - 32 = 1.8C        Subtract C from both sides.
-32 = 1.8C - C
-32 = 0.8C             Divide by 0.8 on both sides.
-32/0.8 = 0.8 C / 0.8
- 40 = C

So that means - 40C = - 40 F