1, as long as the weight is not light enough, then one of the ten piles must be defective. Since you can't see it, you can't weigh it out at once. This solution is only 1 0.9 kg = 900g, 1 kg =1000g. So, you add up to 9900g. Then, divide the 10 pile of jelly by 10, and it is not1000g. It must be defective!

You can weigh 5 packages per side, and the lighter ones are defective. Then, if you divide it again, you can find out ~