I saw the graph that should have accompanied this problem.
y axis = weight in pounds ; intervals of 30.
x axis = number of bags, intervals of 1.
Since the graph forms a line, the rate of change is constant. y = kx
x y
1 30
2 60
3 90
4 120
5 150
Rate of change = weight(lb) / bags = 30/1 or 30 pounds per bag.