calculating dive tables

anyone out there know how to calculate dive tables?
When I am calculating for maximum time on multiple dives I am always coming up with an answer that is 10 minutes more than the "correct answer". what am i missing?

example question: NAUI
A diver dives on a shipwreck at 84 feet for 18 minutes. She takes a lunch break for an hour and 30 minutes. Then she does a dive to 52 feet for 15 minutes. After a 2 hour surface interval, how long can she dive to 44 feet?
Answer:
at 44 feet can stay 41 minutes (29 minutes RNT)... I am getting the 29 minutes RNT but isnt that 51 minutes Maximum Dive Time?

 

Don't have my tables close but are you rounding every depth up? 52ft being a 60 ft dive for table purposes and whatnot?

 

just dug up my tables and my math agrees with you. 51 minutes adjusted dive time. D class diver after every SIT. Book/instructor made a mistake?

 

yes I am... 52ft = 60ft and rounding minutes up as well.
The practice questions are from a separate online source so I am starting to think that these practice questions might be using outdated numbers.

 

my tables are copyrighted at 1989. Could they be using much older?

 

i have no clue... my instructor supplied my table and its copyright 1990.
But if you and I are both getting 51 minutes then I am going to lean towards the question being wrong.

 

even if you reverse engineer the problem there is no class of diver that can accumulate 39 minutes of residual nitrogen time at any depth. Must be a mistake for sure.

 

thank you very much for your help.
I have the test tonight so I am a bit nervous for it!

 

Good luck tonight amigo! Just remember to take your time on it. There are no trick questions and the underwater swim test is more mentally taxing than the written portion is so be confident tonight.