Sum-Things Missing Puzzle
The Puzzle:
 | An old Mathematics book contained this addition sum which had been marked correct by the teacher: |
The most obvious place to start was the 0 on the bottom (don't forget to carry the 1!).
Working towards the left, the next one has to be 9 to make the total 16 (+1 = 17) so we carry another 1.
That means the top left digit must be empty (or a zero):