Answer:
Donna earned a profit of $925, so she sold $925 / $9 profit per bag = 102.78 bags.
She gave away 11 free bags, so she actually sold 102.78 bags + 11 free bags = 113.78 bags.
113.78 bags / 3 bags per set = 37.92 sets of bags.
Therefore, 37.92 sets of bags * 2 bags per set = 75.84 bags were sold in sets of 2.
Therefore, 113.78 bags - 75.84 bags = 37.94 bags were sold individually.
Therefore, 37.94 bags were bought by customers who bought only one bag.