Hi,

I need a very through explanation to the following math questions.... I need you guys to use the "counting principle to answer it."

1. The possible four digit telephone number extensions that can be formed if 0,8,9 are excluded as first digit?

2. Two digits are formed, with replacement, from digits 0 through 9?

a) How many two digits numbers are possible?

b) how many two even numbers are possible?

c) how many numbers divisible by 5

d) how many divisible by 3?

e) how many greater than 37?

f) how many are less than 59?

Answer for 1, it says 7000. I truly don't know what I need to do do here. If we exclude those three does that means we still have 1,2,3,4,5,6,7 #'s that could take #1.

Answers for 2, are a)90, b)45, c)18, d)30, e)62, f)49

This is what I understand, if they say we have to give an award, 1st, 2nd, and 3rd place, but we have 5 people to give the award to, so I understand that it's 5*4*3, because 5 people can get 1st place, leaving only 4 people for 2nd place, and then leaving 3 people for 3rd place.

How does this principle fits in the above problems? I see that there are two digits, but it says "with replacement...." so it cannot be 10*9 = 90 like it says in the back of the book, if with replacement it can be 10*10, because 0-9, is 10, and there is with replacement.

Would it be that the answer key is wrong?

Thanks!!!