ANSWER

2:29

83 feet for 18 minutes -> Group F
Looking at Table 3, using the 60 foot row, looking at the bottom number going from right to left, until we find a number that is 30 minutes or larger, we see that the diver must be a group C diver before she can do this dive safely.
Using the Surface Interval Table, to go from Group F to Group C takes 2 hours and 29 minutes