Hi Carole and thanks for a good network diagram question.
The short answer to your question is that it the duration of any activity in a network diagram is equal to EF-ES. (Early Finish minus Early Start). In your particular question, the activity duration is 10 (EF-ES).
The long boring answer is; if the activity in question is on the critical path then its duration can also be calculated from LF-LS (Late Finish minus Late Start). This is because activities on the critical path have zero slack time. I hope that makes sense.