Creating Dynamic Dependent Lists

I need to create a dynamic lists. I have three cells, the first two cells have a drop down list of choices from 0% to 100% (in 5% increments). The third cell is forced to a value of 100%, depending upon what was selected in the first two cells (e.g., A1=25%, A2=65%, A3 goes to 10%). The TOTAL of the three cells must equal exactly 100% and there can be no negative percentage (e.g., -20%). So if a user choose A1 to be 30%, the drop down menu choices for cell A2 dynamically change to reflect 0% to 70%. If a user chose 45% for A1, the menu choices for A2 would dynamically change to 0% to 55%. If a user chose 100% for A1, the only choice for A2 would be 0%. Does anyone know how to do this? Can you give me the formula? Thanks.

Dependent Validation Lists

Excel -- Data Validation -- Create Dependent Lists

Try the attached workbook.
The sheet contains some explanation.

Wow! I'm a glutton for frustration, anxiety, and exhaustion... but even I could not deal with this anymore. Thank you so much for your help!

(I'm not even going to ask how quickly you did this. I don't want to know!)