For loop in 15 min increments

Hi I am trying to run a for loop to generate a List<String>  for a dropdown list that will allow users to select a time value based on 15 min increments .25, .50, .75, 1.0, 1.25   etc... this is what I have so far but it keeps returning "0" 's
protected List<String> fill_ddlDuriation() { List<String> lstDuration = new List<String>(); for(int i = 0; i < 720; i += 15) { i = i / 60; lstDuration.Add(i.ToString()); } return lstDuration; }