1. Efficiency is defined as work-out/work-in. Muscles operate with an efficiency of about 20% in converting energy internally into work externally. Accordingly, how much energy will be expended by a 60 kg person in the process of ascending several flights of stairs to a height of 25 m? 2. I was using Wg=mgh 3. I used the formula to get an answer of 14715 J, but I'm not sure where to go from here.