When does Celsius and ferenhite is equal?

Answer 1

The relationship between Celsius and Fahrenheit is:
c = (5 / 9) * (f - 32)

So if they are equal, we can simply note that c and f are equal in that equation, and then solve it. Let' call them both "t":

t = (5 / 9) * (t - 32)
∴ t = (5t - 160) / 9
∴ t = 5t/9 - 160/9
∴ t - 5t/9 = -160/9
∴ 4t/9 = -160/9
∴ 4t = -160
∴ t = -40

So -40°F = -40°C