Let’s assume the length of the shortest ribbon is 4x cm. According to the given ratio, the length of the second ribbon is 3x cm, and the length of the longest ribbon is 5x cm. We are given that the length of the shortest ribbon is 12 cm, so we can set up the equation: 4x = 12 Dividing both sides by 4, we get: x = 3 Therefore, the length of the longest ribbon is 5x = 5 * 3 = 15 cm. So, the answer is 15 cm, not 20 cm.