ABCD ABDC ACDB ACBD ADBC ADCB BADC BACD BCDA BCAD BDCA BDAC CABD CADB CBAD CBDA CDAB CDBA DABC DACD DBCA DBAC DCAB DCBA 4 factorial = 4 x 3 x 2 x 1 = 24
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern R-DBAC-. That is, seven letter words with 1st letter R and 3rd letter D and 4th letter B and 5th letter A and 6th letter C. In alphabetical order, they are: redback
let a,b,c,d represent the 4 books abcd, acbd, abdc, acdb, adbc, adcb, bacd, badc, bcad, bcda, bdac, bdca, cabd, cadb, cbad, cbda, cdab, cdba, dabc, dacb, dbac, dbca, dcab, dcba 24 arrangements answer can be found by multiplication select 1st book -- 4 choices 2nd book -- 3 choices 3rd book -- 2 choices 4th book -- 1 choice 4x3x2x1 = 24