BCD addition of three digit numbers

Question

9999+3578

 1 0 0 1   1 0 0 1   1 0 0 1   1 0 0 1
+0 0 1 1   0 1 0 1   0 1 1 1   1 0 0 0

I don't know how to perform addition for larger numbers.

 1 0 0 1    1 0 0 1    1 0 0 1    1 0 0 1
+0 0 1 1    0 1 0 1    0 1 1 1    1 0 0 0
-------------------------------------------
 1 1 0 0     1 1 1 0  1 0 0 0 0  1 0 0 0 1

This is what i am getting

Answer 1

Your problem is that you cant present each digit in binary and sum that up, you need to write down the number as whole in binary, and then sum them up regularly modulo 2.