What is the temperature in degrees fahrenheit of a freezer kept at -20 degrees celsius?

-4 degrees Fahrenheit.

Here's the equations used to convert between the two scales:

C = (F - 32) × 5/9 
F = C × 9/5 + 32

We're converting from Celcius to Fahrenheit, so we'll be using the second one.

F = C × 9/5 + 32 (initial equation)
F = (-20) × 9/5 + 32 (plug in the Celcius value)
F = -36 + 32 (multiply)
F = -4 (add; final answer)