Summary Field - Convert Number of Minutes to Number of Hours

I have a summary field that contains a number of minutes, I want to convert it to number of hours. For example, if it contains 90, I want to display 1.5 Hours.

I tried to create a fomula field like the following, but it does not work. MyTable.MinutesOfPlay is the number of minutes for each play, not a sum of MyTable.MinutesOfPlay for the group. How can I get the group total and put it into the fomula. Thanks.

numberVar totalHours := {MyTable.MinutesOfPlay} / 60;

CStr(totalHours) + "Hours";