The key to this question is that the length of a side of a triangle is larger than the difference of the other 2 sides and is smaller than the sum of the other 2 sides. Hence the length of AC is larger than (or equal to, in case A, C, D do not form a triangle but are on the same line) and is smaller than (or equal to) .
to find the total cost (c) of the colored strips we have to multiply the number of strips (n) by the cost of each color strip (99¢), that as an algebraic function is:
[¢]
Now, if we use 3 solid color strips, that means that we use 4 patterned strips, so: