Answer
We need to pull down on the rope with a minimum force of $480~N$
Work Step by Step
Let $T$ be the tension in the rope. From the diagram of the lower pulley, we can see that at constant velocity, $2T = mg$, where $mg$ is the weight of the box.
Then: $~T = \frac{mg}{2}$
To keep the box moving with a constant velocity, we need to pull on the rope with a minimum force of $T$:
$T = \frac{mg}{2}$
$T = \frac{(98.0~kg)(9.80~m/s^2)}{2}$
$T = 480~N$
We need to pull down on the rope with a minimum force of $480~N$
