The mean temperature for the first 4 days in January was - 2°C.

The mean temperature for the first 5 days in January was - 4°C.

What was the temperature on the 5th day?

-12 degrees.

Step-by-step explanation:

We remember that to calculate the average, we must add all values, then divide by the amount of values.

This means the first 4 days sum to - 8 while the first 5 days sum to - 20. Therefore, on the fifth day, the temperature was - 12 degrees.