General Studies I
In how many different ways can four books A, B, C and D be arranged one above another in a vertical order such that the books A and B are never in continuous position?
else solve it by hit and trial starting with the book A at the bottom various ways the books can be arranged is ACBD, ACDE, ADBC, ADCB
with book B at bottom the various combinations are BCAD, BCDA, BDAC, BDCA
starting with book C at bottom various combinations are CADB. CBDA
starting with book D at the bottom various combinations are DACB, DBCA
Count them all 4 + 4 + 2 + 2 = 12