Q: An activity has an ES of day 3, an LS of day 13, an EF of day 9 and an LF of day 19. What is the activity's duration. 

The given answer is: LS-ES = LF-EF = 13-3 = 19-9 = 10. (PM Fasttrack 7, id 454)

My doubts is that the above formulas calculate the float, not the activity's duration, while the duration is calculated as EF-ES or LF-LS (used in the critical path drawings). So the duration would be 9-3=19-13=6.

Can anybody confirm or clarify my doubt? Thanks in advance.