percentiles

I need to calculate the time for which 90% of responses lie. The data is in two columns like this:
Minutes Number of Instances
1 5
2 3
3 1
4 1

For this example, 90% of the responses are 3 minutes or less.

(Note that the number of minutes are single digits and the number of instances are single digits. I can't get the spacing quite right on the post)

I don't think the percentile function will help me -- at least not as I'm trying to apply it.

Note that I'm generating the base table from a pivot table of "times" and "counts of times". I could set up the pivot table differently if this would help solve my dilemma.

Any ideas?

Thanks much!