Let the efficiency of humans be h and the efficiency of robots be r.In the first case, Total work = (15h + 5r) * 30......(i)In the second case,Total work = (5h + 15r) * 60......(ii)On equating (i) and (ii), we get (15h + 5r) * 30 = (5h + 15r) * 60Or, 15h + 5r = 10h + 30rOr, 5h = 25rOr, h = 5rTotal work = (15h + 5r) * 30 = (15h + h) * 30 = 480h Time taken by 15 humans = days= 32 days.Hence, option C is the correct answer.