main topic covers section C ,ques in management of receivables

Dear sir
can you suggest key area that come up in section C. I know section A & B covers entire area of the syllabus .
I am following Becker revision kit, in that two diff questions’s answers are in the same method. Plz help me

company offers discount @2% for payment within 10 days or full payment within 30 days , assuming 360days a year, what is the annual % cost of the co not taking the discount.
Ans :36.7% (2.04% *360/20)

i calculate as 2/98=.0204 +1
1.0204^360/20= 1.438 = 43.8%