Polynomials of what degree satisfy f^(n) = 0 ,explain your reasoning

So you know that the total width of the posters is 4 + 4 + 4 so 12 inches.
you also know that the ratio of the total poster width to the width of the advertisement is 4:5
so to find the width of the whole board you do
12 / 4 = 3
3 x 5 = 15 inches

then you know that
4t = 15-12
4t = 3
t = 3/4 inches