I have two objects A & B, where the relationship is 1:N

object B has a formula field that calculates hours worked based on a start and end time.

In object A I want to create a formula field that calculates the sum of the 'Hours_worked' field of object B.

This doesn't seem to work though when using:

#CALC_SUM.R32206( Hours_worked )

Is there another way to get this sorted?