Answer: The price of the tied good is $20.
Explanation: The practice of tying is used to package products in such a way that the price of the tied (combined) good is closer to the buyers total willingness to pay for the two goods.
In this case, the total willingness to pay of Carnivore is $20+$7=$27
While, that of Leafygreens is $8+$12=$20
Thus, the producer will sell the combined good at $20 as it this price both the consumers will buy the tied good. If the producer sells it at $27, then only the Carnivore will buy the good but Leafygreens will not.
Thus, with zero marginal cost of serving additional consumer it is better for the producer to sell at $20.