How to find number of rows that meets cumulative percentage critieria

Hello. I have a dataset in Excel 2007 where I need to determine the number of census tracts (as rows) it takes for each column of data values (populations of different ethnic groups) to reach a cumulative percentage of 50% of their City group total. This is a measure of concentration that will vary from group to group.

I can do this manually by sorting each column from largest population percentage to smallest population percentage, then shift-click selecting the first row, scrolling down until I see the sum of percentages shown in the bottom right corner first reaches 50.000% or more.

Is there a cell formula I can use to determine the number of rows it takes to meet the criteria? I have to do this for 250 columns.

Is there a cell formula I can use to determine the number of rows it takes for a column of data values to reach a specific amount, that is first reach a cumulative percentage of 50.000% of the column total? It must be based on the data values from largest to smallest but I don't want to have to manually sort each column I have.

I need to do this for 250 columns so any help to make this easier would be appreciated.