Answer:
To completely fill the sandbox will cost $56
Step-by-step explanation:
First, we calculate volume of the rectangular-shaped sandbox:

We know that density of sand is 100 pounds per cubic foot. Then, to fill the sand box that has a volume of 8 cubic foot, we calculate by rule 3:
1cubic foot..............100 pounds
8 cubic foot.............x pounds


We will need 800 pounds of sand to fill the sandbox, then we can calculate by rule 3 the number of bags needed and next the total cost of them:
50 pounds.......1bag
800 pounds.....x bags

To calculate the cost:
1 bag ..................$3.50
16 bags.................$x

To completely fill the sandbox will cost $56
Answer:
8 is subtracted from x
The inverse operation is addition
Add 8 to both sides
X=-2 so I think that it is: 10 is added to x
The inverse operation is subtraction
Subtract 10 from both sides
4 is multiplied by x
The inverse operation is division
Divide 4 from both sides
X is divided by 2
The inverse operation is multiplication
Multiply each side by 2
Hope it helps!