# Thread: Ok what am I doing wrong here? 2 questions.U

1. ## Ok what am I doing wrong here? 2 questions.U

Hello.
This is under probability in my book where you can use counting or combo or permutation rule.

QUESTION 1
HOW many 5-digit USA numeric postal(zip) codes are possible if the same 5 digits can be repeated?

If this is the counting rule. I get 3125???

QUESTION 2
How many different 3-digit ID tags can be made if the digits can be used more then once??
So n=10, r=3 10Cr = 120 ANSWER in book is 1000
Part B of this question says If the 1st digit must be a 5 and repeats are NOT ALLOWED?
n=10, and reduce r by 1 so r=4?
10P4 = 5040 ANSWER is book is 72.

What am I doing wrong? Why on earth is this so hard I don't get it. I LOVE MATH. Stats as I see is another story.
Any help would be great,
thanks
Jo

2. ## Re: Ok what am I doing wrong here? 2 questions.U

Hey Jo.

I have same problem with statistics and probability. anyways...

q1) Uses the Multiplication Rule: Consider each number on the plate as a position, or a bin to put a number into. How many numbers can you choose from to put in each position/bin?
I get (10)*(10)*(10)*(10)*(10)

q2)
(a) same again: (10)*(10)*(10)

(b) here the first position must be 5 --> 1 choice
second position can be any of remaining 9 digits --> 9 choices
third position can be any of remaining 8 digits --> 8 choices
Mulitiplication Rule: (1)*(9)*(8) = 72.