A climber's gravitational potential energy increases from 14000J to 21000J while climbing a cliff. She expends 18000J of energy during this activity. What is the efficiency of this process? A) 3% B) 39% C) 61% D) 97% Here is what I did. Wpg = ∆Epg = Epg2-Epg1 = 21000-14000 = 7000J % Eff = 100%(useful out)/total in = (100%)7000/18000 ≈ 39% The answer key says it is 61% though. Is the answer key wrong or did I miss something?