Excel Formula Question – Autofill

Hi there,

I’m a grad student doing a project using weather data and i need urgent help on calculations with my data in excel please.

I have wind speed data for every 10minutes in a column but i need to work out the average for every half an hour. I started off using the AVERAGE function e.g. =AVERAGE(C1:C3) and the next one would be =AVERAGE(C4:C6). I tried to autofill this formala but it didnt work as I wanted it too.. Any ideas please?! It is going to take a very long time to do it manually!

THANKS!

Sarah