Using the digits 0 through 9, find out how many 4-digit numbers can be configured based on the stated conditions:
a. The number cannot start with zero and no digits can be repeated.
b. The number must begin and end with an odd digit. (Repeated digits are okay.)
c. The number must be at least 5000 and be divisible by 10. (Repeated digits are okay.)
d. The number must be less than 3000 and must be even. No digits may be repeated in the last 3 digits. (That is, 2234 would be okay, but 2334 would not be okay.)
Please provide an explanation for your solution.