If there are 6 periods in each working day of a school, then the number of ways that you can arrange 5 subjects during the working day is

$$\begin{gathered} \mathrm{Number} \mathrm{of} \mathrm{subjects} = 5. \\ \mathrm{Number} \mathrm{of} \mathrm{periods} =6. \end{gathered}$$

$$\begin{gathered} \mathrm{After} \mathrm{alloting} \mathrm{one} \mathrm{subject} \mathrm{to} \mathrm{one} \mathrm{period} \\ \mathrm{one} \mathrm{period} \mathrm{is} \mathrm{left} \mathrm{over}. \mathrm{From} 5 \mathrm{subjects} \\ \mathrm{select} \mathrm{one} \mathrm{subject} \mathrm{for} 6^{\mathrm{th} }\mathrm{period}. \end{gathered}$$

$\begin{array}{rcl}\mathrm{Number} \mathrm{of} \mathrm{ways} \mathrm{of} \mathrm{arranging} & =& {}^5\mathrm{C}_1\left(\frac{6!}{2!}\right)\\ (\mathrm{since} \mathrm{a} \mathrm{subject} \mathrm{is} \mathrm{repeated} \mathrm{twice})& =& 5\left(\frac{720}{2}\right)\\ & =& 5\left(360\right)=\boxed{1800}\end{array}$