How to solve this problem on alligations with a precision?
After each sale, the milk concentration is $.9$ times what it was previously. We have $.9^6=.53>.5$ and $.9^7=.49...<.5$ so this milkman (who should definitely be in jail) can make sales with milk at concentration $1,.9,(.9)^2,...,(.9)^6$. That's $7$ customers.