Answer:
9 notebooks would cost $6.75
Explanation:
How I solved this problem was by taking the data and writing it out first.
During one trip, he got 2 boxes of pencils and 8 notebooks for $11.00
During the second trip, he got 3 boxes of pencils and 5 notebooks for $11.25
This led me to believe that the boxes were definitely more expensive than the notebooks, as there were many notebooks bought during the first trip, yet that trip was cheaper than the second. So, by the process of elimination, I started off the value of the notebooks to be $0.50, and the boxes of pencils to be $3.00, and I continued to experiment with values until I found two values that would work in both scenarios, the values being $2.50 for a box of pencils, and $0.75 for a notebook. (I know there's likely an easier way of doing this, but I haven't done a problem like this in a while, and this method works just fine. It's just a bit more time consuming.)
1st Case: 2(2.50) + 8(0.75) = 11
2 times 2.50 equals 5, and 8 times 0.75 equals 6. 5 added to 6 gives you a sum of 11.
2nd Case: 3(2.50) + 5(0.75) = 11.25
3 times 2.50 equals 7.50, and 5 times 0.75 equals 3.75. 7.50 added to 3.75 gives you a sum of 11.25
Now that it is known how much each notebook costs ($0.75), you would take that value and multiply it by 9, since you're wanting to know how many 9 would cost, and this gives you a product of $6.75.
I hope this helps :)