Calculate the perimeter of a rectangle if its width is 2.8 decimeters and its length is one-fourth of its width.

To find the length, we will use the proportioning. Let’s denote by x the length. The width is designated as 100%. 1/4 is 25%.
Let’s make the following proportion:
2.8 dm. = 100%;
x dm. = 25%;
Let’s solve this proportion using the method of diagonals, multiplying each diagonal with each other.
100 * x = 25 * 2.8;
Let’s sort this equation by performing the possible multiplication actions:
100 * x = 70;
Find x:
x = 0.7 dm.
The perimeter is the sum of the lengths of all sides.

Perimeter = 2.8 + 2.8 + 0.7 + 0.7 = 7 dm.

Answer: the perimeter is 7 dm.