Write an equality to describe the situation.
A sleeping bag will keep a person warm in temperatures down to -20 degrees farenheit.
To write an equality to describe the situation, we can use the concept of temperature range. Let's denote the temperature in degrees Fahrenheit as T, and use the expression S(T) to represent the sleeping bag's ability to keep a person warm at that temperature.
So, to describe the situation, the equality can be written as:
S(T) = true, for T >= -20 degrees Fahrenheit
This means that the sleeping bag's ability to keep a person warm is true (or satisfied) when the temperature T is greater than or equal to -20 degrees Fahrenheit.