"How many 3-digit numbers can you make with the digits ___...?"
"How many numbers have digits that are consecutive..?"
"How many 2-digit numbers do not contain the digit 0...?"
"What is the probability that a random 3-digit number has a zero as a digit...?"
...then after awhile (maybe after making some mistakes along the way!), you learn to be prepared to watch for the tricky zero digits. A number can't start with 0. It's something good to keep in the back of your head!
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