Repeat the original message after the true message and the check digit.
Eg,
1110 is the original
11101 is the original and check
111011110 is the original and check and original
Therefore we can split this up,
1110 1 1110
if each number is changed:
1111 1 1110
Therefore, we look at the 1st original message and 2nd original message, if they are the same, then this is the correct message. If not, then look to see the amount of 1s in the first 5digits; if it is even then the first 4 digits are the correct message, if it is odd then we know that this has been altered and we look to the final 4digits for the correct answer.
So in this example, there are five 1s in the first five digits so therefore it must be wrong and the last 4digits is the correct message.