Subtracting from total in Numbers? HELP!

I'm trying to create a formula that will take the values of each cell & subtract them from a total value In a cell at the bottom of my spreadsheet (hopefully automatically as I enter a new value in each cell). Basically what I'm wanting to do is keep track of how many more hours i need to broadcast in order to reach the monthly contract requirement of 30 hours.

Thank you!

P.S. I asked this same question earlier this year, and upon reviewing the answer someone provided, I have NO clue how I was able to do it, because the answer makes no sense to me now. I should've saved the document from before!