Answer:
3x - (15 x 3) = 90
or
3x - 45 = 90
Step-by-step explanation:
There are 3 boxes. We don't know how many are in them yet, so we write this as 3x. However, we know that we give away 15 to 3 different teachers. 15 x 3 = 45
Therefore, the first part is 3x - 45
We also know that in the end we have 90 left.
So our final equation is:
3x - 45 = 90