Answer:
First, let's write our equation in a general way.
We know that:
1 cubic foot weighs 80 pounds.
The container weighs 40 pounds.
Then, if i put n cubic feet of sand in the container, the total mass will be one time the mass of the container plus n times the mass of one cubic foot of sand.
We can write this as:
w(n) = 40lb + n*80lb
This is a linear equation, where the y-axis will represent the mass in pounds and the x-axis will be the number of cubic feet of sand in the container in units, a graph of this is shown below.
You can see that it is really steep, this is because for really small increments in n, the weight increases a lot.