The length of the rectangle is 4 times its width, and its perimeter is 210 cm. Find the sides of the rectangle.

Decision:
Let’s designate the width as X cm, then the length will be 4X cm.
Knowing that p = (a + b) x2.
We get the equation:
(X cm + 4X cm) x2 = 210 cm.
5X cm * 2 = 210 cm.
5X cm = 210 cm / 2.
5X cm = 105 cm.
x = 105 cm / 5
x = 21 cm.
Bottom line: if the width is 21 cm, then the length is 21×4 = 84 cm
Check: (21 + 84) x2 = 210 cm.