Answer:
84
Step-by-step explanation:
Since, the digit should be in ascending order, therefore we have to group 123456789 into 3–3 groups with its digits being in ascending order.
<em>Step 1:</em>
Select 1 as one digit
<em>Step 2:</em>
Select 2 as a 2nd digit.
<em>Step 3: </em>
We have 7 numbers (3,4,5,6,7,8,9) as a 3rd digit of a 3-digit number. Therefore, We have 7 numbers in the form of 12_.
Go back to Step 2 and Select 3 as a 2nd digit. Then, we have 6 numbers (4,5,6,7,8,9) as a 3rd digit of the form 13_.
Continue these steps till you get the number of 3-digit numbers in the form of 1__ to 189 which will be 28 numbers.
Go back to Step 1 now and select 2 as a 1st digit and in Step 2, start with 3 as a 2nd digit.
In step 3, you’ll have 6 numbers (4,5,6,7,8,9) as a 3rd digit of the form 23_.
Continue these steps till you find the number of the 3 digit number of the form 2__ to 289 which will be 21
Repeat the steps again to find the number of 3digit number of the form 3__ to 389 which will be 15.
Then find 4__ to 489, 5__ to 589, ……till 789
the answers will be like this
1__=28
2__=21
3__=15
4__=10
5__=6
6__=3
789 which is 1
Adding all these, you’ll get 84.
