How would I calculate CG in this practice problem?
Not too much theory, just simple math.  You find the spot where the sum of all torques 
(mass x distance) = zero.  Where you would balance the plane on a see-saw. That is 
your CG

If you start out with 2300 lbs at 95.75 inches and remove 170 lbs at 87.00 inches, the torque
of 170 x 8.75 = 1487.5 inch pounds has been removed.  The new weight is 2130 lbs.

The new CG is 1487.5 inch pounds/2130 pounds = 0.70 in the OTHER direction.
So new CG is at 96.45 inches.