Answer:
More than 7
Step-by-step explanation:
Given that:
Joshua's goal is to sell more than 20 items at a farmer's market.
Number of items already sold = 6
So, number of more items to be sold > Total number of items to be sold - Number of items already sold > 20 - 6 > 14
Given that each customer buys 2 items.
Let
be the number of customers.
So, number of items bought be the customers = Number of items bought by each customer multiplied by the number of customers = 
And as per the question statement,
Number of more items to be sold > 14 (as calculated in above step)

Therefore, the answer is: <em>More than 7 additional customers are required</em>.