So first let's find the volume of the sand in the sandbox. You have a square sandbox, with a side length of 5, meaning you have a length and width of 5. You also have half a foot of sand in the sandbox, meaning that you have a height of 0.5. To find the volume of the sand, multiply the length by the width by the height:
So you have 12.5 cubic feet of sand. If each cubic foot of sand is 100 lbs, then you just multiply the two:
I dont know if this could help you but if you can please try to answer those two that I didn't complete so you could have an idea on what there relationships are and things like that.