Answer:
For this one I assigned y as the missing number
We know that $6 magazine plus $9 times a specific amount would equal $78
so 6 + 9(y) = 78
6 + 9(8) = 78
9(8)=72
72+6=78
So there were 8 binders bought at $9 each and a 1 magazine bought for $6.
Explanation: