When Arthur comes to Buster's condo to tutor him on homework and advancing his grades, the cup of supplies is on Buster's desk for most of the scene in his room. After Buster stands up from his desk and his friend confronts him for having barely read a chapter, the cup of supplies is gone and Arthur's arms are planted across Buster's desk.
Even though Francine and Muffy teach Buster about dividing 20 pieces of double-dip peanut fudge candies evenly by three, 20 of anything cannot be divided evenly by three. According to a calculator, 20 ÷ 3 = 6.6 (repeating number behind the decimal) estimated to 7 or would equal a mixed number of 6 2/3.
In the school cafeteria, while Fern is drinking juice, the straw is not going through her lips.