Both formulas are correct (if my memory hasn't failed me). Sometimes the 'k' value is given in base e, sometimes it is given in base 10.
Which base is used determines what formula you should use.
I also used Integrated Solid Waste Management - Tchobanoglous/Theisen/Vigil 1993 during the exam.
Both books are still for sale in the yard sale section.
Anyone interested in taking part in an Engineering Boards Fantasy Football league?
If there's enough interest, I'll go through the process of setting one up.