There are 6 periods in each working day of a school. In how many ways can one organize 5 subjects such that each subject is allowed at least one period?
3200
1200
1800
1900
Explanation:
5 subjects can be arranged in 6 periods in $^6P_{5}$ ways.
Any of the 5 subjects can be organized in the remaining period ( $^5C_{1}$ ways).
Two subjects are alike in each of the arrangement. So we need to divide by 2! to avoid overcounting.
Total number of arrangements
$\dfrac{^6P_{5}\times^5C_{1}}{\left(2!\right)}$
=1800