Final answer:
After dividing 240 erasers by 8 to find the total number of boxes and then subtracting the 21 boxes sold, we find that 9 boxes were not sold. This suggests there might be an error in the provided options.
Step-by-step explanation:
The student's question involves a basic division and subtraction problem in mathematics. Mr. Hi packed 240 erasers into boxes of 8, and we need to find out how many boxes were not sold if 21 boxes were sold. First, we calculate the total number of boxes by dividing the total number of erasers (240) by the number of erasers in each box (8).
240 erasers ÷ 8 erasers/box = 30 boxes
Next, we determine the number of boxes not sold by subtracting the number of boxes sold (21) from the total number of boxes (30).
30 boxes - 21 boxes sold = 9 boxes not sold
So, the answer is that 9 boxes of erasers were not sold. This is not one of the provided options, suggesting a potential error in the question choices or an error in the initial data.