The perimeter of the triangle is 20 cm. The lengths of its two sides are 7 cm and 8 cm. Find the length of the third side.

Let us determine with the help of how many centimeters it will be possible to represent the total length of the two sides of our geometric figure, if from the condition of the task we know that the length of one is seven centimeters, and the length of the other is eight centimeters:

7 + 8 = 15.

Let us determine with the help of how many centimeters it will be possible to represent the length of the unknown side of our geometric figure, if it is known from the condition of our task that its perimeter is equal to twenty centimeters:

20 – 15 = 5.

Answer: 5 cm.