Answer
$w_a=71.7\ Btu/lbm$
Work Step by Step
From table A-17E:
Inlet ($T_1=2000°R$): $h_1=504.71\ Btu/lbm,\ P_{r_1}=174.0$
Outlet ($P_{r_2}=P_{r_1}\times P_2/P_1=87.0$): $h_{2,s}=417.3\ Btu/lbm$
From the energy balance for the turbine:
$\dot{W}=\dot{m}(h_1-h_2)$
The actual work is then:
$w_a=\eta\times(h_1-h_{2,s})$
Given $\eta=0.82$
$w_a=71.7\ Btu/lbm$
