Repeat group

Hi,

I am working on form which has repeat group. Inside repeat group I have
first name and last name fields. In the parent form I want first row value
in calulated field. I tried using /data/repeat_group[1]/first_name but it
is not returning value to the calculated field.

It will be helpful if there is some tutorial about how to deal with repeat
group item based on item index to show on calculated fields.

Thanks,
Vipin