有一條PP 唔明!!
我有STEPS但唔明!!
Ten seats are arranged in a row for 10 students from 3 different schools.There are 2 students from school A, 4 from school B and 4 from school C. Assume that students from the same school are indistinguishable.
a) Find the number of ways in which these 10 students can take the seats.
b) Find the probability that the 2 students from school A are sitting next to each other.
a) 10!/(2!4!4!) <------點解要除(2!4!4!)???
b) 9!/(4!4!)/3150<---點解要除(4!4!)???
諗咗好奈!!
幫幫我啦!!!!
THANKS A LOT!!!! :)
更新1:
但點解係除唔係減既???