Step by step
The question
Question: find the surface area of a box 2.5 long, 2 wide and 4 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 2.5 by 2.
Its area is 2.5 times 2.
The bottom is the same.
Two faces: 2 times 2.5 times 2.
Forget the points: 25 times 2 is 50.
The numbers have 1 digit after the point.
So the answer has 1 too: 5.0, which is 5.
So one face is 5.
2 times 5 is 10.
3 Front and back
The front is 2.5 by 4.
Its area is 2.5 times 4.
The face behind it is the same.
Two faces: 2 times 2.5 times 4.
Forget the points: 25 times 4 is 100.
The numbers have 1 digit after the point.
So the answer has 1 too: 10.0, which is 10.
So one face is 10.
2 times 10 is 20.
4 The two sides
The side is 2 by 4.
Its area is 2 times 4, that is 8.
The face behind it is the same.
Two faces of 8: 2 times 8 is 16.
5 Add all the faces
Add the three pairs: 10 + 20 + 16.
10 plus 20 is 30.
30 plus 16 is 46.
So the surface area is 46 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.