Computer Architecture:

6. You ask your friend to think of a four-digit (decimal) number abcd, reverse it like dcba, and subtract the one from the other (taking the absolute value of the difference). Then vou ask him/her to leave out one of the non-zero digits of the difference, and calculate the sum of the remaining digits (of the difference). When your friend tells you this sum, you should be able to figure out the digit that was left out. How? Hint: A (decimal) number is divisible by 9 if and only if the sum of its digits is divisible by 9