Does it matter what interval you use when you find the rate of change of a proportional relationship

Proportions are the comparison of two equal ratios. Therefore, proportional relationships are relationships between two equal ratios.  So yes, it does matter.. Interval: all the numbers between two given numbers.

A. 15% of $20000 is 15/100x20000 = $3000. You therefore get $3000 off the $20000 asking price. $17000

b. VAT is 10%. 10/100x17000 = $1700
You therefore add $1700 to the discounted price of $17000. $18700

c. 15% off the discounted price. 15/100 x17000 = $2550. New discounted price is $17000-2550= $14450.

d. 10% VAT of $14450 is 10/100x$14450 = $1445. Total price after all discounts and VAT is now $14450+$1445 = $15895