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 ?