Let the length of the shortest ribbon be 4x. Then the length of the second ribbon is 3x. And the length of the longest ribbon is 5x. Given that the length of the shortest ribbon is 12 cm, we have 4x = 12. Dividing both sides by 4, we get x = 3. Therefore, the length of the longest ribbon is 5x = 5 * 3 = «5*3=15»15 cm. Answer: \boxed{15}.