Hi there! The unit rate per carton is $4.
To find our answer we must divide the total cost by the total cartons bought.
$ 48 / 12 = $4
Answer:

Step-by-step explanation:
The information that we kow about the shape of the aquarium:
length:

width:

when the reef is submerged, the water level rises 3 feet. The volume that those 3 ft represent in the aquarium will be the volume of the reef.
Thus, we calculate the volume of water "addition" with the same length and width as the aquarium but with a deep of 3 feet (thi is because the reef caused the water to rise 3 feet), and this is the volume of the reef that was added to the aquarium:

The volume of the stone is 
Examples :
2/x < 4 ....multiply both sides by x, cancelling it on the left side
2 < 4x ...now divide by 4
2/4 < x
1/2 < x
5/x + 7 > = 12
5/x > = 12 - 7
5/x > = 5 ....multiply by x, cancelling it on the left
5 > = 5x...divide by 5
5/5 > = x
1 > = x