There are 7 periods in each working day of a school. In how many ways can one organize 4 subjects such that each subject is allowed at least one period?  Pick one.
A) 15345
B) 15120
C) 15609
D) 15680
E) None of these
6 subjects can be arranged in 7 periods in 7P6 ways.   Remaining 1 period can be arranged in 6P1 ways.   Two subjects are alike in each of the arrangement. So we need to divide by 2! to avoid overcounting.   Total number of arrangements = (7P6 x 6P1)/2! = 15120   ANSWER:15120
<result>B</result>
There are 7 periods in each working day of a school. In how many ways can one organize 4 subjects such that each subject is allowed at least one period?