If it counts as "one weighing", he can keep tossing bags on the scale, one at a time, until the total goes up by 9kg instead of a full 10kg.
Edit: decimal point
It has to be something like this:
- number the bags left to right
- take from each bag, the amount of items that the bag is numbered with
- weigh all the items you removed (ie, it will be less than the total amount added up, as one is lighter)
- subtract that from the total weight of all bags
- the answer will be a number, indicating which numbered bag contained the fewer items
It's the only weigh () I see this to be doable?!?