If a skydiver weighs 800 n, what would air resistance be when he is at terminal velocity?

Let his weight be mg.

For a falling body in a fluid.

W - V - U = ma.    Where
                             W = Weight = 800N.
                              m = mass
                             V = Viscous Force = Air Resistance.
                             U = upthrust = Negligible for air.
                             a = acceleration
                             g = acceleration due to gravity.
Note also at terminal velocity  a = zero.
 
W - V - 0 = m(0)
800 - V  = 0
800 = V. 

Therefore the viscous force which is equivalent to air resistance is also 800N.
Cheers.

The rearrangement of particles in a physical change. hope this