The width of a rectangle is one-fifth as much as the length and the perimeter is 120 cm, how do you find the length and width?
Questions of these type are often called Worded Problems.
This is because the key equations that you will use, are already described for you in the question.
Here's how it goes:
Let the length be
The question says: "the width
In mathematical terms this means,
Secondly: "the perimeter
The Perimeter of any rectangle
Now, all that is left to do is to solve
And You're Done.