Let the number of pencils in each box be x.
Total number of boxes is 3.
Total number of pencils in 3 boxes = x+x+x=3x.
Given His mother gives him 4 more pencils. Now jack has 28 pencils
Or 3x+4=28
To x term 4 is added To isolate x term we subtract 4 both sides .
3x+4-4=28-4
3x=24
To x ,3 is multiplied .Opposite of multiplication is division.
Dividing by 3 both sides
x=8.
Number of pencils in each box is 8.