Answer:
The temperature on the 5th day was of -9ºC.
Step-by-step explanation:
Mean of a data-set
The mean of a data-set is the sum of all values in the data-set divided by the number of values.
The mean temperature for the first 4 days in January was 1°C.
This means that during the first 4 days, the sum of the temperatures was 4*1 = 4ºC.
The mean temperature for the first 5 days in January was -1°C.
First 4 days: Sum of 4º
5th day: Temperature of x.
The mean is -1, so:
The temperature on the 5th day was of -9ºC.