JEE MainMathematicsPermutations and CombinationsNumerical+4 / −1
The number of integers, between 100 and 1000 having the sum of their digits equals to 14 , is .
Numerical answer
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Correct answer: 70
Let the three-digit integer be , where:
- is the hundreds digit, so
- are the tens and units digits, so
We need:
We must count the number of digit triples satisfying these conditions.
1. Remove the lower bound on
Since , define Then and .
The equation becomes:
Now we count non-negative integer solutions of with digit restrictions:
2. Count all non-negative solutions without upper bounds
The number of non-negative integer solutions of is
3. Subtract invalid cases
We now subtract solutions where any variable exceeds its allowed maximum.
Case 1:
Let Then Number of solutions:
Case 2:
Let Then Number of solutions:
Case 3:
Similarly, number of solutions:
4. Check overlaps
- and would require at least , impossible.
- and impossible.
- and impossible.
So there are no overlaps to add back.
5. Final count
Thus required number is
Therefore, the number of three-digit integers whose digits sum to is
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