Step by step
The question
Question: find the surface area of a box 5 long, 5 wide and 2 high.
1 Count the faces
The surface area is the area of all the outside faces.
A box has 6 faces.
They come in 3 pairs, the same size.
2 Top and bottom
The top is 5 by 5.
Its area is 5 times 5, that is 25.
The bottom is the same.
Two faces of 25: 2 times 25 is 50.
3 Front and back
The front is 5 by 2.
Its area is 5 times 2, that is 10.
The face behind it is the same.
Two faces of 10: 2 times 10 is 20.
4 The two sides
The side is 5 by 2.
Its area is 5 times 2, that is 10.
The face behind it is the same.
Two faces of 10: 2 times 10 is 20.
5 Add all the faces
Add the three pairs: 50 + 20 + 20.
50 plus 20 is 70.
70 plus 20 is 90.
So the surface area is 90 square units.
Let's look back.
First: count the faces.
Then: top and bottom.
Then: front and back.
Then: the two sides.
Last: add all the faces.