Query regarding costing and idt preparation.

I have started preparing for Group-2 and i have picked up AMA and IDT for nov'12 exams at first.

For costing i am using tulsian's book and he has given 1 full chap on pcc's Marginal costing where BEP/MOS and all are given....i don't know will it be asked in exams?

Or should i goto Decision making staright away?

In IDT i have started with Excise, but its of 300 pages and i dunno what is important  for exams.........cause there are lot of rules and laws given.

Do advice.