The perimeter of the rectangle is 20cm, and the sum of the lengths of its three sides is 14cm. What is the area of this rectangle?

1) Subtract from the perimeter of the rectangle the sum of its three sides and find the value of the fourth side: 20 – 14 = 6 (cm);

2) Since the sides of the rectangle are pairwise equal, we calculate the doubled value found from the sides: 6 · 2 = 12 (cm);

3) Subtract from the perimeter the doubled value of its two equal sides and determine what the sums of the other two equal sides are equal to: 20 – 12 = 8 (cm);

4) Find out the size of the other side of the rectangle: 8: 2 = 4 (cm);

5) Using the formula, we calculate the area of the rectangle: S = a b = 4 6 = 24 (cm ^ 2).

Answer: S = 24 cm ^ 2.