The temperature at which Fahrenheit and Celsius thermometers read the same number is -40 degrees.

Here's why:

* Fahrenheit to Celsius Conversion: The formula to convert Fahrenheit to Celsius is: C = (F - 32) * (5/9)

* Celsius to Fahrenheit Conversion: The formula to convert Celsius to Fahrenheit is: F = (C * 9/5) + 32

If we set these equations equal to each other (F = C) and solve, we find that the only solution is -40 degrees.