Shop A: 20% discount then 9% GST on a $500 watch.<br><br>
&bull; Discounted price = $500 × (1 − 0.20) = $500 × 0.80 = $400<br>
&bull; After GST = $400 × 1.09 = $436<br><br>
Shop B: $450 flat (GST included).<br><br>
&bull; Shop A ($436) is cheaper than Shop B ($450).<br>
&bull; Difference = $450 − $436 = $14<br><br>
Shop A is cheaper by $14.<br>
Check: $400 + 9% of $400 = $400 + $36 = $436. ✓<br><br>
Final Answer: Option D: Shop A is cheaper by $14