Ned asked each of his 4 children to think of a 4-digit number.
"Now transfer the last digit to the front and add the new number to the old one.
For example, 1234 + 4123 = 5357.
Now tell me the results."
The results were told by each of the four children as follows:
Barry: 2348
Mary: 7847
Jaypee: 11847
Darla: 9846
"Only one of you gave me the correct sum" Ned told the gathering.
Who was it and how did Ned know?