Question 311385
A rectangular solid has three dimensions.
Length = L
width = W 
Height = H
.

The volume of a solid is given by L*W*H
For example . The L=6 inches, W= 4inches,H= 5 inches

Then the volume of the box = 6*4*5=120 cubic inches.
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Now you can visualise that any box, carton has six faces.
Place a carton or box in fron of you. You will observe
The opposite two faces are always equal..
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The face which is the base of the box has an area of L*W
The opposite face that is on top also has the same area. = L*W
Therefore tput together he two opposite faces have an area of 2*l*W
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One face on the side will contain W&H 
So area of two opposite faces 2*W*H
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The other two face will conatin the sides H & L
So the area of the opposite sides  will be 2*H*L
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SO the total area of all six sides of a box = 2*L*W + 2*W*H+ 2*H*L square units.
If a box has dimensions of 3 4 & 5 
Then the total surface area = (2*3*4) + (2*4*5)+(2*5*3)
=24 +40+30
=94 square units