In our optimal solution, we are giving the maximum allowed dose to the spinal cord (5). If we were to relax this, how much could we decrease the objective? Change the right-hand-side (RHS) of the spinal cord constraint to 6, and re-solve the model. By how much did we decrease the objective? (Hint: the previous objective value was 22.75)
If you change the RHS of the spinal cord constraint to 6 and re-solve the model (Tools->Solver, then hit solve) the new objective is 22.1666667. So we decreased the objective by 0.58333333.