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Correct answer: 569
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We need to count all 4-digit integers in the interval using only the digits
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Since the number must lie between and , let the 4-digit number be . We count valid numbers casewise by the thousands digit.
Case 1: Thousands digit =
Then the number is of the form . To be in , it must satisfy
All digits must come from .
Total numbers starting with :
- : choices
- : choices
- : choices
So total is
But some are less than . Since the first digit is already , compare with :
- If , then compare the last two digits with .
- If , then any gives numbers . This contributes invalid numbers.
- If , then we need to be valid. So only is invalid. This contributes invalid number.
- If , then number is already .
Thus invalid numbers below are
Hence valid numbers in this case:
Case 2: Thousands digit =
Any number of the form is automatically between and .
Choices for each of are each, so count is
Case 3: Thousands digit =
Now the number is of the form , and must satisfy
We count valid possibilities by comparing with .
Subcase 3.1:
Allowed digits less than from the set are So has choices. Then can be anything from the set: choices each.
Count:
Subcase 3.2:
Now compare .
So we need Both digits must be from .
- If , then since available digits for are all from the set, all possible are less than . For each such , any is allowed. That gives
But when would need , impossible since is not available. So no extra restriction arises.
Thus count here is .
Hence total for thousands digit is
- Therefore total count is
So the required number of integers is
Comparison with stored answer
Stored correct answer = .
Our derived answer is also , so they agree.
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