Take 1 bag from 1, 2 bags from 2, 3 bags from 3 and 4 bags from 4.
10 packages are weighed together. If the weight of each bag of washing powder is 500g.
There are 5000 grams.
Because there is a box of washing powder with only 450 grams per bag, it will be less.
If it is only 4950g, and there is (5000-4950)/50= 1 bag, then 1 box of washing powder is unqualified.
Similarly, you see how many 50 grams are missing from 5000 grams, and which box of washing powder is unqualified.