Here is the problem, am bit confused why they did not use Pa-Pb in the solution. I know in abc seq if the voltage coil does not go with seq letter then that wattmeter reading should be negative. Per than for Wa is between a and b so should be positive but Wb is between c and b instead of b and c...
z in p.u = actual value/ base Value, then if you assume base as 1 MVA then how you did 1/40 should not it be 40/1, another question how MVA/MVA will be Z it will be MVA in p.u. Correct me if am wrong.