Adding Drop-down Values to column/Running Total

[powerwindow]

I am trying to create a way to keep a running total of values selected from a drop down list. I have a drop down list of values increasing in 5 step increments. How can I select the value in the Drop-down and have that value added to a running total? I'm trying to achieve this to use on the mobile Excel app so I cannot use vba. Otherwise, I had seen how I could make a nifty button increase values, but can't use buttons in mobile Excel.

Basically I want to select numbers from a single drop down and have that number added to a sum that can be shown in a different cell.

Thanks for any tips.