Find the temperature when the fahrenheit scale reading is exactly equal to the celsius scale .

The temperature is -40 degrees, this means that:

-40°C = -40°F

Explanation:

If we have a temperature T in the Celcius scale, the equation to transform it into the Fahrenheit scale is:

(T*9/5) + 32

Now we want to have the same reading in Celcius than in Fahrenheit, then we must have:

(T*9/5) + 32 = T

We need to solve this for T.

T*(9/5) - T = -32

T*(9/5 - 5/5) = -32

T*(4/5) = -32

T = -32*(5/4) = -40

This means that -40°C = -40°F

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.