The perimeter of the rectangle is 30 cm. What are the length and width if the width is 4 times less.

From this task it follows that the perimeter is 30 cm. And the length is 4 times greater than the width. So it turns out the following:
Since the length is 4 times the width, the length is 4 parts and the width is 1 part. Let’s count how many parts make up the entire perimeter of the rectangle. Since the perimeter is equal to the sum of all sides, that is, we add two lengths and two widths, we get the following:
4 + 4 + 1 +1 = 10
That is, the entire perimeter consists of 10 parts. We find out how many centimeters 1 part is, for this we divide the perimeter by 10, we get the following:
30 ÷ 10 = 3 cm
This means that one part is 3 cm, that is, the width is 3 cm, and the length is 3 × 4 = 12 cm.