This is under probability in my book where you can use counting or combo or permutation rule.
HOW many 5-digit USA numeric postal(zip) codes are possible if the same 5 digits can be repeated?
ANSWER is 100,000.
If this is the counting rule. I get 3125???
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,