In a group of 3 girls and 4 boys, there are two boys B1 and B2. The number of ways, in which these girls and boys can stand in a queue such that all the girls stand together, all the boys stand together, but B1 and B2 are not adjacent to each other, is :
(4) 144
Two blocks (all girls, all boys) 2!=2 ways; girls inside: 3!=6
Boys, B1B2 not adjacent: 4!-3! 2!=12
2 6 12=144
Answer: (4)
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