In triangle ABC, side AB is 27 cm and it is 3 times larger than side BC. Find side AC if the perimeter of ABC is 61 cm.

Let’s solve this task by actions with explanations.
1) 27: 3 = 9 centimeters – the length of the BC side, since it is 3 times less than the length of the AB side, and its length is 27 centimeters;
2) 9 + 27 = 36 centimeters – the sum of the lengths of the sides AB and BC, since the length of the side AB is 27 centimeters, and the length of the side BC is 9 centimeters;
3) 61 – 36 = 25 centimeters – the length of the AC side, since the perimeter of the ABC triangle is 61 centimeters, and the sum of the lengths of the BC and AB sides is 36 centimeters.
Answer: 25 centimeters.