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.

Given that the length of the shortest ribbon is 12 cm, we can set up the equation: 4x = 12

Dividing both sides of the equation by 4, we get: x = 12/4 x = 3

Therefore, the length of the longest ribbon is 5x = 5 * 3 = 15 cm.