Final answer:
The tax per pack of seltzer is $8.
Step-by-step explanation:
The tax on a pack of seltzer can be calculated by finding the difference between the price paid by consumers and the amount received by producers. Before the tax, consumers paid $9 per pack and producers received $4 per pack. After the tax, consumers pay $12 per pack and producers still receive $4 per pack. Therefore, the tax per pack of seltzer is $12 - $4 = $8.