The average monthly income of A and B is ₹3,590.

$ \frac{A + B}{2} = 3590 A + B = 7180 $
$ \frac{B + C}{2} = 4250 B + C = 8500 $
$ \frac{A + C}{2} = 6500 A + C = 13000 $
Now add all three equations:
(A + B) + (B + C) + (A + C) = 7180 + 8500 + 13000
2(A + B + C) = 28680
A + B + C = 14340
Since B + C = 8500
A = 14340 - 8500 = 5840
Since A + C = 13000
C = 13000 - 5840 = 7160
Answer = 7160