First, figure out how many bags of potatoes would be sold if six bags of onions were sold. The ratio of onions sold to potatoes sold is 6:1. So, if he sells 12 bags of onions, we must divide this number by six to get the number of bags of potatoes sold: 2 bags
Then, use this number to figure out how many bags of carrots he would sell:
The problem tells us that he sells two bags of potatoes for each bag of carrots. Therefore, he would sell 1 bag of carrots if he sold 12 bags of onions.