Question on sales tax with discount calculation

Dear Sir,

As I am unable to find “ask a tutor” link on F3 portal I am presenting my query here…
Can we calculate the sales tax related problems by deducting just the trade discount first & adding the sales tax on the amount we get as the net receivable which is the assured amount as we are not always sure if the cash discount will be applicable or not.

For eg. in Test Q 1 on Sales Tax of Chapter 12 of notes it says goods of $ 480000 sold on credit to John. 10% trade discount with additional 5% cash discount if paid within 14 days. But it is unclear if or not it is been paid within 14 days…Sales tax applicable is 17.5%

if we deduct both the discounts & then add the sales tax it is as good as assuming amount with cash discount for sales tax add up…

ie if amount isnt paid within 14 days cash discount becomes ineffective & sales tax will be on 480000-48000= 432000+ST= 507600

the cash discount is applicable only if the amount is paid within 14 days!
480000-48000=432000-Cash Disc of 5% = 410400 + ST = 482220

But the ans given for receivable is after calculating the trade & cash discounts together & then adding the ST & reimbursing (negating the cash discount effect which comes to 482220 + 21600=503820….!

It must be obvious that if we add back the cash discount the Sales tax for the same must be added too… ie extra 3780$ (17.5% of 21600)