No of course days offered (Southcott-Activity Based Costing)

Question : Firm offers short revision courses on taxation and auditing. Annual overheads $152,625.Until recently firm has applied overheads on volume basis,based on number of course days offered.Only consultant own time in direct variable cost.All students run with maximum number of 30 students. Following are the information. No of courses sold Duration of course Auditing 50 2 Taxation 30 3 Problem: 1) I cannot understand what is number of course days offered actually. 2) How do we calculate OH per course based on traditional method.