Subtracting from total in Numbers?

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 hours i need to broadcast in order to reach the monthly contract requirement of 30 hours.

Thank you!

P.S. I should probably mention I've never used formulas in Numbers or Excel before so I'm definitely new to this.