Answer:
17
Explanation:
let x be the number of boxes to be shaded to make it 80%
x = 80% of total number
= 80/100 of 25
= 80/4
= 20
So 20 need to be shaded to fill 80%
Now, 13 of the grids are already shared so the amount of more squares that need to be shaded = 20-13
= 17 squares