FA BPP Revision Kit 2020-2021, Incomplete Records, 20.3 & 20.7 , Pg 97 , 98

Greetings sir, In 20.3, the calculation is based on the rule that gross profit is always 40% of the sales revenue. Sales margin calculation in short. My issue: Why I cannot use the doubtful sales revenue to calculate the doubtful COS and then subtract it from the Actual COS mentioned in Question, in order to find the difference of theft ? METHOD A 181600 (doubtful sales) x 60/100 = 108,960 (doubtful COS) (Actual COS) 114,000 - 108,960 (doubtful COS) = 5,040 difference Even if I use the gross profit, it comes the same: METHOD B (Gross Profit) 67,600/181,600 (doubtful sales) x 100% = 37.225% 40% - 37.225% = 2.775% 2.775% / 100% x 181,600 = $5,040 The answer however is $8,400 which is calculated using COS: METHOD C 114,000(Actual COS) x 100/60 = (Actual sales) 190,000 - 181,600 (Doubtful Sales) = $8,400 However in Q.20.7, when I use the same METHOD A, it appears to be the only way to get the right answer (sale margin being 25%. Therefore: 612,000(sale) x 75/100 = 459,000 (COS of sales margin). 516,000(COS of Op.Inv+Purchase-Cl.Inv) - 459,000 (COS of sales margin) = Ans) 57,000 Inventory difference.) So why in Q 20.3 this method fails (calculation from sales to COS) and in Q20.7, this method wins? Is it because in 20.3 Sales is doubtful, so we use COS to Sales as a base for calculation, and in 20.7 COS is doubtful, so we use Sales to COS as a base ? If I am correct, could you please elaborate more ?