11 ⅓ gallons.
Step-by-step explanation:
Since she sold 3⅓ gallons more on Sunday than she did on Saturday, you need to add 10⅔+3⅓. ⅓+⅔=3/3, or 1. 10+3=13, and 13+1=14. She sold 14 gallons on Sunday.
On Monday, she sold 2⅔ less than she did on Sunday, so we need to do 14-2⅔. 14-2 is 12, and 12-⅔= 11⅓. This means that she sold 11⅓ gallons on Monday.
Hope this helps :)