a. There are two ways to select the first digit, 1 or 2, and 10 ways to select each of the next three digits, hence:
You can do your own arithmetic.
John
My calculator said it, I believe it, that settles it
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(a) The solution is VERY SIMPLE.
Under given condition, the numbers are integers from 1000 to 1999, inclusive.
In all, there are exactly 1000 (one thousand) of such numbers.
(b) As the problem is posed, it is unclear to me, what restrictions work in this case.
If you mean odd integer numbers of the category (a), then their amount is half of the 1000 numbers of the category (a), i.e. 500.
If you mean all odd four-digit numbers, then the amount of such numbers is