Take 1 coin from the first machine, 2 from the second, 3 from the third and so on. So there will be 55 coins. Weigh them. If all the coins were normal then the weight would have been 550 grams but since some coins are defective the weight may be more or less. We get the following cases.
Case-1:
The weight is greater than 550 gram.
This
means that one machine produces heavy coins. Calculate the amount by which
the weight exceeds 550 gram. If it exceeds by x grams then xth machine
is defective.
Case-2:
The weight is less than 550 gram.
This
means that one machine produces light coins. Calculate the amount by which
the weight is less than 550 gram. If it is less by x grams then xth machine
is defective.