Spreadsheet Formula Determining Average Increase

I am needing help determining how to write a spreadsheet formula that would determine the average increase of a number over time. For example:

Day 1: 10,000

Day 2: 20,000

Day 3: 30,000

Each number represents the number of times a certain action has taken place since starting Day 1. I need to find a formula needed in order to determine the average increase between each day and convert that formula into a spreadsheet formula so that all I need to do is put the new total for that day into the spreadsheet. Obviously, the average increase in this example is 10,000 actions/day, but I am dealing with a lot more random numbers and I am averaging the number of times these actions occurs each day by inputting the number of times the action has occurred altogether since Day 1.

Obviously, it would also be easier for me to just input the increase in the amount of actions per day and just have it add them all up and figure the average, but that would leave me to do the simple math of taking the new days number and subtracting from it the day before 50 times everyday for a very long time. That is why i am trying to find a formula that I can input into a spreadsheet that will pull the increase and average the daily increase by simply inputting the new total over the entire time period.

I hope I made sense in my explanation. Don't hesitate to ask questions if you do not fully understand what it is that I am needing.