The mean temperature for the first 4 days in January was -11°C.
The mean temperature for the first 5 days in January was -13°C.
What was the temperature on the 5th day?
the mean temperature is -11 for the first four days, meaning their sum must be -11*4=-44C from that, we know -44+x=-13*5 now we solve for x x=-65+44 x=-21