How many 4-digit numbers are there …
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Posted by Js Dear 8 years ago
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Amar Kumar 8 years ago
Let 4 digit number be abcd
The choices do we have for the digit a
We have 9 choices (since the first digit cannot be 0).
Now for each possible choice of a we have 9 choices for b (since we want b to be a different digit to a and we now allow 0 ).
So for choice of the ab part we have 9∗9=81 possibilities.
Now for each of these we have 8 choices for c (to avoid c being the same as either a or b). For each of these we have 7 choices for d (to avoid d being the same as either a,b or c ).
So in total there are 9∗9∗8∗7=4536 possible numbers.
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