The conversation between human and AI assistant.
[|Human|] Calculate the discount percentage on this purchase.
[|AI|] The discount percentage on this purchase will be 20%. 

Here's the breakdown of the calculation: 

Discount percentage = (Discount amount / Item cost) * 100%
Discount percentage = (30 / 150) * 100%
Discount percentage = 0.2 * 100%
Discount percentage = 20%
[|Human|] 
Calculate the discount percentage on this purchase.