SOLUTION: A four-digit whole number has four different odd digits. The number 3 appears in the hundreds place. What is the biggest number this could be.

Principle: Get the biggest possible digits as far left in the
number as possible.

To make it the largest possible, obviously the first digit must
be 9.  We have no choice for the hundredth digit because that's 
given to be 3. Then we take the largest remaining odd digit to
put for the ten's digit, and that's 7, the next odd digit down
from 9.  Then the next largest remaining odd digit for the one's 
digit is 5.  So the answer is:
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9375
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If you had been asked for the smallest it would have been 1357.
Principle: Get the smallest possible digits as far left in the
number as possible
Edwin