A thermostat registers temperature error as E = |T - 20| where T is the temperature in °C. If the error is 5°C, what possible temperatures T satisfy this?

T = 25°C or T = 15°C

Every temperature between 15°C and 25°C inclusive.

T = 5°C or T = 35°C

Only T = 25°C because temperature cannot be less than 20°C in this setup.