triangle BAC is just a larger version of triangle EAD. given that ED is 5cm, and BC is 10cm, triangle BAC must be twice the size of EAD. since the ratio of AD to ED is 4:5, and BC is twice the size, all we need to do is multiply both sides by 2 to find length AC. 4*2 = 8. now we can subtract length AD from AC to get CD. 8 - 4 = 4.
