Show correctness of a condition in a task in the hall of 8 rows. on 9 chairs in each row. To the hall imparted pupils of two kalass. on 30 pupils in everyone.

In the hall of 8 rows on 9 chairs in everyone. 2 classes in each of which on 30 people came into the hall. Learn how many chairs will remain a svobodnymia a question to a task where
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