Question

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?

Answer 1

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

The temperature for the fifth day is -21°C

Answer 2

Answer:

I have no idea how to do it either but at least I can get some points