Question: how many 5 digit combinations are there using 0-9?
Answer: The trick is to realize that a number can not start with a zero! Now, there are 105 ways in which the digits 0-9 can be chosen for the five places of a five digit number. Out of these, 104 start with zero (once we start with 0, there are only 4 slots to fill, where we have 10 choices each).