Excel formula to recalculate averages?

I created an Excel workbook to keep track of work projects, and my team has to work a certain number of hours per month on each project. I'd like to create a worksheet in the workbook that displays the average number of hours to work each month that auto-updates after every month to display the new average number of hours needed to stay on target.

In my spreadsheet, I'd like to be able to input the total number of hours worked so that it then displays the number of hours we'd have to work every month for the rest of the year to meet that number.

To keep things simple, let's say I have twelve rows--each one representing one month for a year-long project. If my team has been assigned 120 hours over the year to work on the project, that means by default we should spend 10 hours per month. If in the first month of January I manually enter 20 hours as the amount of time we spent on it, the "average hours to work" column for February through December should be approximately 9.1 hours each.

I can't quite think about the best way to do this and would appreciate any insight. Thanks!