Answer
The maximum number of passengers is 28.
Work Step by Step
Since the speed is constant, the upward force exerted on the elevator is equal in magnitude to the weight. Note that the speed $v = \frac{20.0~m}{16.0~s} = 1.25~m/s$
Let N be the maximum number of passengers.
$P = F~v = 40~hp$
$(600~kg)(9.80~m/s^2)(1.25~m/s)+ N~mg~v = (40~hp)(746~W/hp)$
$N~mg~v = 29840~W - 7350~W$
$N = \frac{22490~W}{(65.0~kg)(9.80~m/s^2)(1.25~m/s)}$
$N = 28$
The maximum number of passengers is 28.