Answer:
36
Step-by-step explanation:
Usually, when we talk about an n-digit number, we mean one that has no leading zeros. If we use all 9 non-zero digits, the single number having increasing digits is ...
123456789
We cannot swap any digits and have the digits remain in increasing order. The only thing we can do to create different numbers is remove different pairs of digits.
There are 9C2 = 9·8/(2·1) = 36 different ways to choose 2 digits from 9. Thus, there are 36 possible 7-digit numbers with the digits in increasing order.
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<em>Additional comment</em>
The first few are 1234567, 1234568, 1234569, 1234578, 1234579, 1234589, ...
