The mean air pressure in Beijing in the first 25 days of September 2015 was 1014.5 hPa. The daily mean pressure for the remaining days of September 2025 is shown below. 26th 1013 hPa 27th 1016 hPa 28th 1025 hPa 29th 1028 hPa 30th 1015 hPa Find the mean air pressure for the whole of September 2015 in Beijing. Give your answer to the nearest hPa.
I have been able to calculate the mean for the last 5 days, which is 1019.4 hPa. But I am stuck on how you incorporate this with the mean for the first 25 days, because if add up all the air pressures and then divide by 30, I get an answer of 204 hPa. The correct answer is 1015 hPa, but I don't know how you get it.
Any help would be greatly appreciated (please explain the process step-by step).
the mean air pressure for the first 25 days is $1014.5$ so multiply this by $25$ to get the total for the first $25$ days. Then add the remaining days' values to get the grand total. Divide the grand total by $30$