Initial equilibrium:
[A]=0.5 M, [B]=0.375 M; Kc=0.50.375=0.75
After adding 0.1 mol A: [A]=0.6 M, [B]=0.375 M; Q=0.625
Q < Kc, so equilibrium shifts right.
Let x be A that decomposes:
0.6−x0.375+x=0.75
0.375+x=0.45-0.75x
1.75x=0.075; x≈0.0429
New [A]=0.6-0.0429=0.557 M; [B]=0.375+0.0429=0.418 M