help -> combinations and permut (1 Viewer)

[Tryingtodowell]

I JUST DONT GET THIS TOPIC hardest ext 1 topic for me

 

[WeiWeiMan]

View attachment 42984
I JUST DONT GET THIS TOPIC hardest ext 1 topic for me

a. 9 choices for first digit (can't be 0), 9,8,7 etc for next digits
total = 9*9*8*7*6*5*4*3*2*1=9*9! = 3265920

b. 9 choices for first digit (can't be 0), 10 for each subsequent digit
total = 9 * 10^9 [9 other digits]