Answer:
If the drum set is 30% off, this means that Susan only needs to pay 70% of the original price. We can calculate this using the following formula:
Total price = Original price x (1 - Discount rate)
where the discount rate is the percentage discount expressed as a decimal. In this case, the discount rate is 30% or 0.3. Substituting in the given values, we get:
Total price = $71.28 x (1 - 0.3)
Total price = $71.28 x 0.7
Simplifying, we get:
Total price = $49.896
However, we also know that Susan got a discount of $21.39. This means that she only needs to pay:
Total price - Discount = $49.896 - $21.39
Total price - Discount = $28.506
Therefore, Susan has to pay $28.506 in total.