1. The problem statement, all variables and given/known data in this question , it's not stated that whether the temperature change from 20°C to -20°C or -20°C to 20°C . I'm confused... 2. Relevant equations 3. The attempt at a solution I think it should be changing from 20°C to -20°C so delta T = (-20-20) = -40°C , am i right ?