the mean temperature for the first 4 days in January was 9°C the mean temperature for the first 5 days in January was 7°C. what was the temperature on the 5th day?

Ok to make this easy, do 4 days as 9°C. So 9+9+9+9=36 / 4 = 9. So that's the first 4 days. Then the 5th day, the mean dropped to 7°C so that means the temperature dropped. -1 is your answer and you can check this by doing 9+9+9+9+(-1) = 35 / 5 = 7°C.