Perimeter of the rectangle = 40 cm. Find the smaller side of this rectangle. If one of the sides is 3 times larger than the other.

Brief notation of the problem condition:
P rectangle = 40 cm.;
a and b ̶ sides;
a> b 3 times;
b =?
Decision:
Let the smaller side of the rectangle be x cm, then the larger side will be 3x cm.The perimeter of the rectangle is 2 * (x + 3x) cm.By the condition of the problem, the perimeter of the rectangle is 40 cm, then we write the equation:
2 * (x + 3x) = 40;
2 * 4x = 40;
8x = 40;
x = 40: 8;
x = 5;
Answer: 5 cm.