Create a new data element in your main data source using the Whole Number or Decimal data type. Place it on your canvas as a text box. Double-click the box and set the default value by clicking the Fx button. Click Insert Function, choose Sum, double-click "insert field here", choose the Hours node of your repeating group, click Filter, and set the filter to when the Category node equals training. This should sum all the hours from the repeating table for that category.