Histogram cumulative 20% of the distribution

Hi all,

I am working with the Analysis ToolPak in Excel 2010. I need to make some histograms that include the following "bin" categories - 25, 50, 75, 100 - but unlike the default setting in Excel, I don't need the bin categories based on the values in the histogram, I need them based on the percentage of the distribution. For example, if my values are 5, 20. 30, 50, 70, 70, 80, 90, 100, I need to know the value at the cumulative 20% of the distribution - what's the value at 20% of the distribution, not the % of cases that have a value of 20 on a scale of 100. Any help would be appreciated - thanks!