As this is a unitless problem, let us try to deal with it as if it does have units. Since we are already given the length and width of a rectangle, we can solve for the area by multiplying. After getting the area, we divide this by the area a wheelbarrow can cover to obtain the expression for how many wheelbarrows we need.
Rectangle length = 10x^2 + 90x
Rectangle width = 5x^2
Rectangle area = length * width = 5x^2(10x^2 + 90x)
Rectangle area = 50x^4 + 450x^2
Dividing:
_5x^2_+_45____10x^2|50x^4 + 450x^2 50x^4 -------- 450x^2 450x^2 --------- 0
Therefore, the expression for how many wheelbarrows are to be used is 5x^2 + 45 wheelbarrows.
