Satya,
I have been looking at this a bit more and I think you are actually correct. Actiivity A has a duration of 10 days and we need to count 3 days "lag" beyond day 10 which takes us through day 13. We count back 9 days from that to determine when activity B started which would be at the beginning of day 5. Total duration for A and B would then be 13 days as you stated. I had to remember that for a FF+3 relationship means that activity B finishes 3 days after activity A. My apology for any confusion as earlier, I had put the lag from the start of activity B and it needs to be added to the finish of activity A.
Was there any kind of Gantt chart or network diagram shown with the answer?
Thanks,
Mark