Answer: 45 markers in each box.
Explanation:
Initially, the school had 3 boxes of board markers, and each box had X board markers, where X is the number we want to find.
Then the initial number of board markers is 3*X.
We know that after giving 15 markers to 3 teachers, we have other 90 markers left. This can be written as:
total markers = markers for the teachers + markers left
3*X = 15*3 + 90 = 45 + 90 = 135
now, we divide both sides by 3.
X = 135/3 = 45
From this, we know that in each box we have 45 markers