Step by step
The question
Question: find the surface area of a box 8 cm long, 5 cm wide and 3 cm 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 8 by 5.
Its area is 8 times 5, that is 40.
The bottom is the same.
Two faces of 40: 2 times 40 is 80.
3 Front and back
The front is 8 by 3.
Its area is 8 times 3, that is 24.
The face behind it is the same.
Two faces of 24: 2 times 24 is 48.
4 The two sides
The side is 5 by 3.
Its area is 5 times 3, that is 15.
The face behind it is the same.
Two faces of 15: 2 times 15 is 30.
5 Add all the faces
Add the three pairs: 80 + 48 + 30.
80 plus 48 is 128.
128 plus 30 is 158.
So the surface area is 158 cm².
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.