40/3 or 13.3
Work Step by Step
Since ADE ~ ABC we can assume that AD=AB and DE=CB. We are also told that DB=BC. We can call BC X. Then we set up a proportion AD/DE = AB/CB. However we do not know the measure of DB or BC. We plug in 8+X for AB and X for CB. The resulting proportion is 8/5=(8+X)/X. You then solve and get 40/3 or 13.3.