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Assume that two particular boys as a single boy and also two girls as a single girl.
so, 4 boys and 9 girls, two boys can also arrange by 2! ways , similarly two girls can also arrange by 2! ways.
number of ways when they are together = 13! *2*2
number of ways when there is no restriction = 15!
required number of ways when they are not together = 15! - 13! * 2*2
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