Take one pill from the first batch, two pills from the second batch, three from the third, and so on until you've taken ten pills from the tenth batch. Weigh.
The total weight of the pills, assuming they are all uncontaminated, is 450 mg (1+2+3...+10). The key here is to see how much heavier the actual pile of pills is compared to the expected value.
For example, if your total weight is 455 mg, that's because five of the pills are contaminated and thus heavier. It therefore must be down to the five dodgy pills from your fifth batch.