What is the net force on a sky diver falling with a constant velocity of 10m/s downward?

If his velocity is constant, then there is zero net force on him . . . any force acting on him is exactly canceled by an equal force in the opposite direction.

In the case of a sky-diver, the force of gravity on him (his weight, downward) is exactly canceled by the force of air resistance upward.

The net force on him is zero.  If the net force on him were not zero, then his velocity  wouldn't be constant. He would be accelerated in the direction of the net force.

acceleration = (change in speed) / (time for the change)

change in speed = (20-120)= -100 m/s.

1.5 min = 90 sec

acceleration = -100/90 = -1.11 m/s^2.

we don't care about the 150kg or the 2,500 meters. they don't make any difference.

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increases it