Final answer:
Luke's calculation shows the actual cost will be more than £300 because the cost per parcel is likely to increase after the first 20 parcels.
Step-by-step explanation:
Luke's calculation shows that the actual cost will be more than £300 because he is assuming a constant cost per parcel. However, it is likely that the cost per parcel will increase after the first 20 parcels. This is because many businesses offer discounts for larger quantities, so the cost per parcel typically decreases as more parcels are sent.