If we had AB: BC: CD, we could easily have solved that problem. In order to combine these ratios, we need to have the same number for BC. We can create this number by finding LCM (Least Common Multiple) for 5 and 3, which is 15. Then, we can write ratios of AB:BC = 6:15 and BC:CD=15:20. Now, we can easily combine these ratios. AB:BC:CD = 6:15:20. Then, 6k+15k+20k = 82 and k=2 cm. And BC = 30 cm
