- #1
LostSeeker
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Homework Statement
----1. From the digits 0, 1, 2, 3, 4, 5, 6, how many four-digit numbers with distinct digits can be constructed? How many of these are even numbers?
----2. How many integers between 1000 and 9999 inclusive have distinct digits? Of these how many are odd numbers?
Homework Equations
Use the product rule, etc.
The Attempt at a Solution
----1. The first question is straight forward.
There are four-digit numbers: _ _ _ _
From left-to-right on possibilities: 6 (can't be zero; hence, 6 versus 7) * 6 * 5 * 4
Answer: 720 possibilities.
The second type of question is where the troubles begin.
Does anyone know of a simple online tutorial that would help? I'm sure this is all obvious but it is not to me.
The following is an attempt. . .
There are four-digit numbers: _ _ _ _
From right-to-left on possibilities: 3 (even numbers), 6 (even or odd), 5 (even or odd), 3 (even or odd but no zero)
Wrong answer: 270 possibilities.
Where am I going wrong?
----2. Again, the first part is easy for the second problem. The answer is 9*9*8*7 = 4,536.
The second part confuses me.
From right-to-left on possibilities: 5 (odd numbers), 9, 8, 6
Wrong answer: 5*9*8*6 = 2,160.
According to the textbook, from right-to-left the answer really is: 5, 8, 8, 7.
Why is this correct? Why those particular numbers?