This does involve a couple of methods, but is achievable. Have a look at example 10 in this form as a demo: http://assure.ly/kUP02y.
// set up a variable
var vTotal = 0;
// resolve the row objects in the table
var nRows = xfa.resolveNodes("Table1.Row1[*]");
// set up a loop to go through the repeating rows
for (var i=0; i<nRows.length; i++)
if (nRows.item(i).CheckBox.rawValue == "A")
vTotal += nRows.item(i).DecimalField.rawValue;
// set total field
this.rawValue = vTotal;
Note that you will have to change the object references Table1, Row1, Checkboc and DecimalField to suit your object names.
Hope that helps,