[tex]\begin{gathered} \text{ For the first book on the line, we have 4 choices,} \\ \text{ For the second, we have three },\text{ since we already chooseed one for the first book} \\ \text{For the third, we have two, by the same reason} \\ \text{ And for the last we only have one posibility} \\ \\ \text{The number of different ways is then} \\ 4!=4\cdot3\cdot2\cdot1=24 \\ \\ We\text{ have 24 ways!} \end{gathered}[/tex][tex]\text{ Remember:}n!=n(n-1)(n-1)\ldots(2)(1)[/tex]
